More Words, Deeper Hole

There were vague plans by the scientists and engineers working on the Manhattan Project to continue research and development of nuclear weapons as a joint US-UK venture with some assistance from Canada; what would have happened to Bohr and the other non-English-speaking types involved wasn't clear. The US political establishment put a kibosh on that with the McMahon act of 1946 and the British were sent home after having their luggage searched and any relevant documents confiscated. Mindwipe and/or a trench out back of one of the buildings in Los Alamos were not an option and Guantanamo Bay was not yet a gulag so the returnees were able to recreate a lot of what was needed from notes and memories plus the records of the original Tube Alloys work done before the US deigned to join in the fight against Fascism in early 1942.

In the end there was some R&D collaboration but at arms length mostly and only after Britain demonstrated their home-grown nuclear capability in the 50s, including a fake "thermonuclear" test meant to give the impression to US observers that they had their own fusion weapon when in fact it was just a very big fission device.

The "Eternity Ring" idea works sort-of if you assume that the research going on in the UK in 1946 isn't the primary target of the spy ring but the the people who worked on the Manhattan Project, a socialist Britain (Health care for all!) being perceived as easier to spy in than "Reds under the Beds" paranoid America.