How did Churchill lose the 1945 after playing an instrumental role in winning the war?

There’s a few distinct aspects to why this was the case, which we can broadly break down into main contributing factors. There was how people saw Churchill himself, what people thought about the Conservative party more generally, and what people were looking for post war.

Churchill was consistently a hugely popular leader. His approval rating from 40-45 never fell below 78% with the British public. But he was seen as a war leader, and his views on peacetime social policy were never particularly highly regarded. For the previous decade he had been a backbench politician until war broke out with Germany, fairly unpopular within his own Conservative party and far from positions of real power.

Which brings us to the point about how people saw the Conservative party more generally. Churchill was the leader of the Conservative party during the war, but the government he led was not a Conservative government, it was a broad wartime coalition. People may have seen Churchill as the best person to lead Britain in war, but this was because of his very outspoken views against Germany in the years before, his predictions of danger being proven correct. The Conservative party, on the other hand, had followed a policy of appeasement, and the British public remembered this. There were a lot of people who, despite liking Churchill, blamed the Conservative party for the policy of appeasement - a policy Attlee and the Labour Party had opposed - and therefore would not vote for the party in ‘45.

As well as many resenting the Conservative party for their inaction against Hitler when Germany was still weak, most of the British public were desperate for social change. This was not something Churchill or the Conservative party were pushing for. Churchill admitted social change was necessary but didn’t think it was the place of the government to bring it about. Labour, on the other hand, were very supportive of sweeping social changes.

The economist William Beveridge published the Social Insurance and Allied Services report in 1942, detailing what he saw as necessary post war social reforms and policies. This report, which became widely known as the Beveridge Report, suggested huge social changes, including social housing (this was of particular importance due to housing shortages caused by German bombing), employment insurance, and the establishment of the National Health Service. The report was incredibly popular - there are instances of people queuing up to buy a copy, which is pretty unheard of for a government report.

Labour took the ideas of this report and ran with many of them as the basis of their manifesto, and these were hugely popular with the British public. In contrast, the conservatives ran on a far less ambitious platform, promising lower taxes, high defence spending and preserving the British empire, with little regard for domestic social change.

The combination of these factors resulted in one of the biggest electoral swings of the century, labour winning 393 seats, an increase of 239, and the conservatives holding only 197, down 189 seats. Labour’s campaign focussing on domestic change had struck a chord with the British people, while the conservatives’ focus on Churchill’s personal popularity didn’t grasp that this wouldn’t necessarily translate from war to peacetime.