There are probably as many as forty people who can lay a claim to owning the title "person who single-handedly won WWII", among them Churchill, Alan Turing (more of which later, probably), General Montgomery, and so on. Memorably, Churchill was the one who provided the isolated British with stirring speeches and "never-surrender" leadership during the dark days of 1940 and beyond, Montgomery led his forces to victory in the Desert War, and Turing was one of the many mathematicians (most of whom came from King's, oddly enough) who helped to break German codes, giving the Allies a huge edge in the war.
One of the more unusual claimants might be Group Captain (Sir) James Martin Stagg, whose job it was to ... predict the weather. Specifically, he was the meteorologist who was working on predicting the weather in early June, 1944, at about the time of D-Day. Sea crossings can be fairly hazardous, and in addition for the Allied Navy and Air Force to do their thing you needed calm seas, good visibility and reasonable cloud cover too. Without this the whole business might not have worked at all, and it was Stagg's job to give the OK to launch the invasion fleet.
The BBC's version of the story goes that the morning of June 6th was the only gap in a nasty weather front that lasted for about two days, and Stagg predicted this gap, allowing the invasion to be launched. The cruncher was that the defending forces did not see this gap coming, so as a result the forces caught the defence off guard in more ways than one.
And the rest, as they say, is history...
[With thanks to the BBC's "Great British Weather" for inspiring this post.]