[Crito]

My understanding is that the US was not particularly open with nuclear technology initially. The UK developed fission bombs and designed thermonuclear bombs more or less independently, demonstrating that they had done that to the Americans, and the Americans then became more open to cooperation.

The US even allowed the UK to test some of their bombs on American soil, in Nevada. (The UK also tested nuclear bombs in Australia. They don't have much wide-open desert of their own).