I answered a question along similar lines that should hopefully give you some context:

https://www.reddit.com/r/AskHistorians/comments/cnf2l1/on_what_grounds_is_churchill_remembered_as_a_good/

EDIT: yeesh, this blew up. Thank you for the kind words and the gold (and the platinum!). I only want to emphasize that the linked post was meant to give some context here rather than be a spot-on answer: it's a reply to a closely related but in the end somewhat differently framed question, after all, so the fit won't be 100%.

I'm going to have to disagree with a few comments here. It seems what you are asking is not what Churchill is remembered for (e.g. fighting on beaches - something which resonated less at the time then it did today according to the Mass Observation reports at the time) but why does he have such a dominant place in the social memory of Britain? This, much like most social memories has much more to do with the politics of the contemporary than it does of the actions of individuals in their time and place. So I would argue to fixate on simply a wikipedia list of 'greatest hits' misunderstands the dynamics of social memory (messy as they are) and the complexities of post-war British identity. Instead, this can potentially imply that his actions (and by extension other major moments who loom large) naturally project onto our consciousness by a historical osmosis created by his evident rightness of action. This strips the complexity and (in my mind beauty) of historical memory into a mere passive jar of an uncritical reading of the past.

While this question is in the perfect venn diagram of my wheelhouse (post-war Britain and social memory/history) I made the decision to abstain from answering this question as a whole after an hour or two of thinking of my answer. The reason being is this is so immensely complicated, subtle with huge variations of era, social group, political alignment, and exact form of the memory. Instead, I can only sherpa a few complexities. Churchill forges a path in the consciousness which becomes pickled in the milleu of the moment and space, not one that transcends it. I could probably just about manage a multiple thread answer about a single key point in this (2nd term, his death, 1983 invocation, statue protest) - but in all history since 1945 ... no.

So I will simply say two things - one as a counterpoint of the other posts here and one to hopefully illustrate the complexity we are dealing with here (I assiduously claim to only be giving a 'casebook' and by no means exhaustive account of the relationship between Churchill and the present).

So firstly. Churchill as revered does not stand as Churchill the man. His political past, war years, initial conservatism, pre-war liberal reforms and his crossing the floor, Tonypandy, Sidney street, gradual estrangement from Liberal politics, WW1 politics (with a minor exception for Gallipoli, and event then not focused on particularly in the UK myth) crossing the floor again, troublesome relationship with things such as the General Strike, India, socialism, his baffling decisions as Chancellor, the complexity of his position is all absent in our conception, involvement in the Norway debacle (so obvious even here on this thread, with Churchill essentially springing from the ground around appeasement and disappearing in the cheers of VE day!), his post-war opposition parliamentarism (which in my opinion is his most interesting time), inanimate second term, awkward farewell and death (indeed - I would be surprised if many individuals who hold dear to this myth would be confident naming the year... or decade ... of his death - because the Churchill we imagine doesn't really have a past after 1945 in this imagination).

No, Churchill, a richly interesting, fascinating, frustrating, contradictory man, whose career is emblematic of a profound half-century of change is reduced to a few pop-culture-esque snapshots of WW2. We are not remembering Churchill the man, nor the leader, but an episodic stint.

This need not be a problem, but is vital to understand as his image and myth is essentially conflated into WW2 and the myths around that. Those myths, sense of a struggle and experience of millions and the man are so tightly interwoven that Churchill has as much a political and social shorthand for an experience, a generation, a place in the world. This is natural to expect, he was PM, but it is vital we understand we are not examining why Churchill looms large, but why His Britain looms as such. This point is essential and why I am adamant against gesturing over to the 'made for TV moments' and saying "because of that". You are not remembered on the actual record of your actions and merits, but the perception of what your record was, something which becomes diluted, mudded, stripped and added to a general conception of the time, its meaning and relation to the present.

This is the thing about the Myth of Churchill, if we compare him to the myth around other PMs: Disraeli, Thatcher, Blair, Attlee - these are all thought in party and policy terms. How they manifested, changed, improved, polluted the creed of their party in the context of a changing peoples. We do not do the same for Churchill. He is not a political figure remembered in this traditional sense. We do not marvel at his fiscal policy, admire his party manoeuvring, his electoral success (his record on elections really is not very good, losing two and losing the popular vote on his sole victory) or ideological platform (the most interesting policies in his second term were the product of the bright and energetic cabinet he assembled, often working around the grand old man). He isn't even really a figure of the party in the way of Baldwin, Thatcher, Macmillan etc. He is above policy. He is so intrinsically associated with the war, and a British self-conception of itself during the war that we essentially throw out anything which does not conform.

This is where my second point comes in. Churchill was in part the architect of this trend to wrap a collective imagination of experience within one man. His major works at the end of the war became the dominant public facing narrative of the war... which unsurprisingly (given the man) is a less than an accurate representation. It is not that he was engaging on a mass deception, but he was the man he was, and his writing was essentially a political memoir. None of this is out of the ordinary. However as a man with the credential to speak of the war, and the lack of other archival evidence released his narrative existed a persuasive grasp of the imagining of the war in its immediate aftermath (and even among historians, including the venerated AJP Taylor who was later to moderate his early effusal). This story captures the national perception of the war of 'principle, idealism and determination, winning out against all the odds' with him at the centre (indeed, in his works you will find it hard to identify the millions of common people involved aside from reinforcing the aforementioned themes of heroic self-sacrifice and bravery or provide amusing anecdotes) it is a story of Churchill - the man and war. This, generally acclaimed upon its release was then made anew for a newer generation in the film 'The Finest Hour' which essentially formed a film facsimile of the themes of the book and became the most successful film in the box office to for its time.

As the immediate experience turned to memory, and turned to distant memory he was then caught up in the sharp politics of the 1970s and 80s. With the left responding the opening up of the archives to show a more complex picture of wartime governance/appeasement, emphasis on colonial legacy, and radicalisations against this myth, in turn pitching an also radicalising right into a culturalist defence and appropriation. Soon, Thatcher was pouring scorn on Attlee 'wasting the peace' (a not so coded attack on the crumbling social democratic consensus he was associated with) and appropriating Churchill in 1983 around the Falklands engagement. He has existed since, as a trophy and icon - not a man of a policy, or ideology - including our current PM going as far as writing a (not very well recieved) biography of the man to associate him with the myth. On the left he has also become a shorthand, particularly in the last decade or so. This is not to suggest that claims against him or examples are invalid, but our imaginations and counter-imaginations tend to reduce and only operationalise parts of his past confirming with the expediency and personal imagination of past-England.

These are but a few examples, he has ceased to be (and one could contend after 1945 never was) a historical PM, or even a man, instead he is an abbreviation of a past England to be moulded and thrown around at one's particular political convenience.

This is essentially why I have an issue with the wording of the proposition. I am not sure 'revering' is the right word to describe Churchill in the mentality of Britain in the post-war era. Certainly there is admiration and respect, but he is not something one thinks about everyday. Nor does his 'creed' shape decisions and conceptions going forward (his neutering politically has seen to that impossibility). I would contend he, and his memory, is more of a cultural 'touchstone', we reference as part of our collective imagined past (with obvious deviations).

None of this is a statement on his character or impute his qualities as a war leader. However, as a historian who loves the complexity of our recent past and the dynamics of our memory of it he stands tall as a gleaming example of all the misrecognitions, shaving of complexities, simplifications, and plain bad histories which obscure this richness on the altar of simplified historical memory.

So, my answer: why does he loom large? It is bloody complicated, and involves a painstaking assessment of the dynamics of memory forming and myth making within the changing post-war British people across dozens of sections and subsections of society. But it is certainly much more complex (and interesting) than the commonly remembered slivers of a wartime governance. It is a story of Britain in the post-war era, with the challenges and complexities since.

Churchill, a fascinating, contradictory, and complex figure has been neutered, reduced, and shorn of his vividness to fit our particular imagined past and anxieties of a lost-England.

I don't have much to add to the brilliant answer of /u/CopperBrook about Churchill as the touchstone for the British self-conception of the war, but do want to touch a bit on the point raised about Churchill's deliberate architecture of that myth since it relates a little to your comparative question about FDR and Truman.

From the day he took office, Churchill had always intended to write a memoir for a very simple reason: writing was how he paid the bills from his days as a columnist onward, and indeed, the wild commercial success of The Second World War was what made him go from struggling as a near spendthrift to very comfortable for the last 15 or so years of his life. In his book In Command of History on the writing of the series (which I'd recommend if you're interested in how Churchill shaped his own historical narrative), David Reynolds estimates that in today's dollars, Churchill made anywhere between $18-50 million on the deal, almost all of it tax free.

But there was another aspect to this. Churchill also knew that whoever was first out with the insider political narrative of World War II would establish it as the base for all other reference materials for decades to come, and once in opposition he had plentiful time to write - and whatever else you can say about Churchill (and there's a lot to be said), the man could turn a phrase.

What he did not expect in 1940, though, was that his work would essentially be the sole insider narrative of the war through the 1960s.

FDR had made noises that he'd write something - he had actually brought George Elsey up to Hyde Park over a weekend in 1944 to establish a game plan for organizing his library - but his death meant that it took decades for professional historians to go through his papers and letters, and even today trying to actually discern what FDR was actually thinking at any given moment in time is often perilously close to a guess. Truman? In an somewhat jaw dropping move that has almost completely been forgotten, while still in office in 1950 he opened up White House, State, and Defense Department records to an outside historian - who at least was told which classified material was off limits - for an authorized biography, partially in response to Jimmy Byrnes slamming him in his own 1947 best selling autobiography. In 1955 Truman did publish the first of his two autobiographical volumes, but by then the narrative had both been set (Churchill's final volume had come out in 1953) and while it sold well, Truman was not only someone who had very little to add until he took office in 1945 but also a vastly unpopular ex-President; it really took until the end of the Cold War and David McCullough's book to reevaluate him as far more than that haberdasher in the White House.

But the lack of insiders went well beyond the leaders (and obviously, the Axis leaders didn't get to write their versions, although plenty of Wehrmacht generals did.) Of the top three underlings in the White House during the war, Harry Hopkins had died in 1946, Bill Leahy wrote a terrible and terribly received book, and Jimmy Byrnes' political hitpiece on Truman had been written with the not-so-subtle intent of trying to salvage a political career that had gone down in flames after being saddled with Yalta. Frank Knox had died of a heart attack in 1944, and while Henry Stimson's 1947 autobiography was (and is) somewhat useful as a limited primary source, FDR had largely ignored him (much like Cordell Hull) so their roles in shaping policy were relatively minimal. All in all, it's no coincidence that up until the late 1960s most historians of those years relied upon actor/speechwriter Robert Sherwood's Roosevelt and Hopkins: An Intimate History as their definitive source material (even though he was out of that job by 1943 and even before that had not been consulted or even been present for many of the major decisions) because there really wasn't all that much available unless you'd started digging through acres of papers.

Of the senior American military officers? Nimitz and Spruance never wrote autobiographies and rarely spoke about the war, Marshall outright refused large sums of money for his own and took until 1955 until he began speaking with an authorized biographer, Halsey's biography sold well in 1947 but even at the time was largely discredited given his bizarre excuses for his mistakes (and later tarred and feathered by Morison for them), King's autobiography in 1952 wasn't all that well received, and MacArthur didn't publish his own autobiography until right before his death in the 1960s, long after his reputation had fallen off a cliff.

This left only one real bit of literary competition: Eisenhower, who'd published Crusade in Europe in 1948 - but while he became somewhat of an insider with the legendary memo to Marshall about Army Pacific operations shortly after Pearl Harbor, his full seat at the table didn't really happen until Torch (and given the chaos around Darlan, how rickety that chair was is a legitimate question.) Even then, though, Churchill was more than cognizant that he might not want to openly criticize someone he thought he might be dealing with down the road as Prime Minister to President - and Eisenhower wrote with the same limitations in mind for his own burgeoning political career.

So in short, because he was not just the first out of the gate but nearly the only significant contributor for years, this non-professional and rather biased historian by the name of Winston Churchill burnished the political figure of Churchill for decades after the The Second World War came out - at least, until the professionals really started digging. It's something to think about regarding why his legacy turned out the way it did.

and was this reputation something believed during the war

Churchill was very popular in Britain during the war. There are a number of sources that attest to this. Between 1940 and May 1945, the polling company Gallup took over 30 polls on the question:

...do you approve or disapprove of Mr. Churchill as Prime Minister?

The results of these and numerous other polls (hundreds and hundreds) were collated and published in The Gallup International Public Opinion Polls: Great Britain 1937-1975: Volume One: 1937-1964 by George H. Gallup (ed.) (New York: Random, 1976). Here are the results for the aforementioned question:

​

Period	Approve (%)	Disapprove (%)	No Opinion (%)
Jul 1940	88	7	5
Oct 1940	89	6	5
Nov 1940	88	7	5
Jan 1941	85	7	8
Mar 1941	88	7	5
Jun 1941	87	9	4
Oct 1941	84	11	5
Dec 1941	88	8	4
Jan 1942	89	7	4
Feb 1942	82	11	7
Mar 1942	81	13	6
Apr 1942	82	13	5
May 1942	87	8	5
Jun 1942	86	9	5
Jul 1942	78	15	7
Aug 1942	82	11	7
Sept 1942	82	10	8
Oct 1942	83	11	6
Nov 1942	91	7	2
Dec 1942	93	5	2
Jan 1943	91	7	2
Apr 1943	90	7	3
Jun 1943	93	4	3
Jul 1943	93	5	2
Nov 1943	91	6	3
Jan 1944	89	7	4
Mar 1944	86	10	4
Apr 1944	88	9	3
Jun 1944	91	7	2
Aug 1944	89	8	3
Oct 1944	91	7	2
Jan 1945	81	16	3
Feb 1945	85	11	4
Mar 1945	87	10	3
Apr 1945	91	7	2
May 1945	83	14	3

Of course the next question is how reliable are those polls. Richard Toye, in his book The Roar of the Lion: The Untold Story of Churchill's World War II Speeches (Oxford: Oxford University Press, 2013) notes that Gallup's operation was not perfect:

​

Commercial pressures forced [Henry] Durant [who ran the Gallup operation in Britain] to make trade-offs between cost and quality, and clients' needs and best survey practice. (p.6)

However I should add that Professor Toye does not believe that the problems with the Gallup polls invalidate their findings completely, and he concedes that Churchill's popularity was "astonishingly high" (p.6) during the conflict. One potential objection to their accuracy might be that if Churchill's popularity was so high, then why did he lose the 1945 election so badly? However, Gallup polls had suggested a very comfortable Labour lead over the Conservatives for several years prior to the general election of 1945. It appears that the electorate liked Churchill but did not like his party.

Another organisation which carried out surveys to assess people's satisfaction with the Prime Minister was Mass Observation. These gave a somewhat lower satisfaction rate, but according to Toye the lowest rate was still 66% satisfaction (in March 1942), so still quite high overall (p.228 & 235). According to Toye though, there is no reason to believe that Mass Observation was more reliable than Gallup. He concludes:

​

perhaps [we] should not worry too much about precise numbers, given that the methodologies of both organizations can be called into question and, either way, he [Churchill] clearly had a very strong bedrock of support. (p.228)

Toye also discusses some evidence that Churchill's wartime speeches weren't always warmly or even overwhelming positively received. For example, A Home Intelligence survey on the impact of Churchill's 'Be Ye Men of Valour' speech (19 May 1940) suggested that about half of respondents were worried and scared by it (p.228).

Sources

George H. Gallup (ed.), The Gallup International Public Opinion Polls: Great Britain 1937-1975: Volume One: 1937-1964 (New York: Random, 1976)

Richard Toye, The Roar of the Lion: The Untold Story of Churchill's World War II Speeches (Oxford: Oxford University Press, 2013)