According to 'The Birth of the Bomb' by Ronald Clark, 1962, the British actually raised the possibility of a uranium bomb in a committee meeting just before Christmas 1939. A second memo in July 1941 commented that nuclear weapons might be possible 'before the end of the war'. In 1942-43, a research establishment was built next to a poison gas factory in Wales to test the equipment needed to refine Uranium. The British played a key role in encouraging the Americans to start their own nuclear program because they had the sense to realise they didn't have the resources to do it alone.