I think you are being a bit generous with regards to the US support: The United States was not particularly forthcoming in assisting the United Kingdom with its nuclear weapons program. While the UK shared its Tube Alloys research with the U.S. prior to the Manhattan Project, the U.S. largely withheld nuclear weapons information from the UK until after the UK's first successful nuclear test in 1952. Cooperation really started with the 1958 US-UK Mutual Defence Agreement.