So the British never had a "development" project to make a bomb. They had an "exploratory" program that was devoted to learning about whether a bomb could be made. In other words, theirs was a program that was concerned mostly with the theoretical side of things, as oppose to a program concerned with the actual development.

The timeline is roughly like this:

• The US starts an exploratory program in late 1939 (Uranium Committee). It doesn't do very much. It is focused mostly on reactors and the possibility of moderated bombs. It does not generate much enthusiasm for a bomb-production project because the physics of a moderated bomb is not very favorable, and the difficulty of enriching material is high. Reactors seem more plausible but to use a reactor for power seems a long way away.

• The UK MAUD Committee starts in 1940. It is motivated by optimistic estimates of the amount of enriched uranium they might need. It correctly identifies (which the US program did not) that a bomb of highly-enriched material would work with a lot less material than an moderated bomb (which won't work at all), but incorrectly identifies the difficulty of enriching material and how much material you'd need (they are overly optimistic). They think it would not be too hard for the US or Germany to pull it off and send a report to this effect to the US.

• The US Uranium Committee head gets the report and shelves it. In mid-1941 the UK sends over an emissary to figure out what the US is doing, finds out they are doing nothing much. The emissary contacts several key scientists in the US and convinces them that it really isn't that hard to enrich uranium. Around this same time, the US isolates the first plutonium and thus has another (reactor-based) way forward. These scientists seize control of the uranium project, rename it (S-1 Committee), and start investigating whether they could really go forward with a full project.

• By 1942 the US scientists are convinced that they could go forward for a full project. Roosevelt approves the measure. They bring in the Army Corps of Engineers to help coordinate and construct the massive facilities needed. By early 1943 they are going at it with a full crash program — produce a bomb at any price. (They were still overly optimistic — it costs 5X more than they expected, took longer than they thought, was harder than they thought; by the time they realized it was much harder, they were already deeply committed, and the secrecy helped insulate them from doubtful eyes that might have canceled it and directed the resources elsewhere.) This is what we call the Manhattan Project, which grew out of the Manhattan Engineer District of the Army Corps of Engineers.

• The UK did continue some work on their side of things, looking at a few specific questions about reactors and gaseous diffusion, but their outlays were pretty small. In 1943, Churchill convinced Roosevelt to share information with the British (against the advice of Roosevelt's advisors, who distrusted the British and didn't think it could help the US project much), and an expedition of British scientists was allowed to work on the Manhattan Project (though they were deliberately cut out of several parts of the project). By this point, the UK was not trying to make any kind of independent bomb. I think with hindsight we can agree with their assessment at the time that they did not have the resources or manpower to do it within the timescale of World War II. As it was, they started their own independent nuclear program in 1947 (after they had been cut out of further cooperation with the US by the signing of the McMahon Act), and set off their first bomb in 1952. Their later program was relatively leisurely in its development compared to the Manhattan Project (5 years versus 2.5 years), but they were not operating under similar time constraints and did not pour as much money into the program as a result (and they also had the advantage of their experience on the Manhattan Project; they were not really doing it "from scratch"). (And, in fact, no other nuclear development program has ever been as fast as the Manhattan Project, in terms of going from "decision to build a bomb" and actually building one.)

So you can see that by the time it is really "the Manhattan Project," it has, almost by definition, overtaken the British program — because the British program was never a production program, and the Manhattan Project designates the phase of the US project that was one. I also like to continue to point out that the British did play a key role, but it was mostly in convincing several key US scientists (Compton, Lawrence, Bush) of their overly-optimistic estimates. The original US estimates on difficulty (as with the German estimates) were more accurate — they reckoned that it would be maybe just possible to make an atomic bomb, but it would be extremely expensive and very risky. As it was, the US managed to make 3 atomic bombs by early August 1945, at 5X the cost that they expected, and only by employing nearly 1% of the entire US civilian labor force to do so (about 500,000 people total). If they had been a couple months delayed (no difficult thing, as anyone who has managed even small projects can tell you), they wouldn't have had a bomb ready to use before the end of World War II. So it was risky.

As for targets — to my knowledge there was no serious target planning on either side until they actually had an idea of when the bomb would be available, what its physical dimensions would be (which tells you about whether you could drop it from a plane, and what kind of plane), and what its explosive power would be (which was uncertain even until 1945). So while I am sure there were very loose ideas about what they would do if they had one, I am not sure they are very meaningful. (The US, for example, first imagined that maybe it would use one against Truk lagoon in 1943, but no truly serious planning was done on this, and by mid-1945 Truk was an irrelevant target.)

I wrote an essay review of a recent book on "Churchill's Bomb" [which you can read here](http://alexwellerstein.com/publications/wellerstein_racethatwasnt(npr).pdf), and it goes over some of these themes in more detail. Here is my conclusion, which puts some of the big points on programs and investment in perspective, I think, and I have highlighted my favorite sentence:

I worry, as a reviewer, about sounding like a chauvinistic American (or, worse, General Groves) when I dismiss the early British “lead,” see the British role in the Manhattan Project as mostly catalytic, and instead emphasize the role of “big engineering” in developing the actual bomb. But this is an important point, for any nuclear program: equations on paper do not an atomic bomb produce. At best, they can give guidance for the future work of making the bomb, or can convince a politician that making an atomic bomb is a worthwhile investment. But without the further investment, no bomb will ever result. The only figure that Farmelo gives for the wartime British project is that, by 1943, the United Kingdom was spending “a quarter of a million pounds a year,” a little over a million US dollars at the time. Though this sounds like a lot of money, during that same time, the Manhattan Project spent considerably more than that sum every single day. Cleverness, and a good start, were perhaps necessary conditions for making the atomic bomb, but they were not sufficient, and never would have been.