You need to read up on Tube Alloys. That's the codename of the world's first nuclear bomb program -- the British one. It got under way via the MAUD committee in late 1940, and in 1943 the allies agreed to a merger -- the UK sent all their scientists and research material over to join the MP (on the strict understanding that the results of the MP would be shared: an understanding which was ignored by Harry Truman once the Manhattan Project delivered in 1945).

Also worth googling: Klaus Fuchs.