The US Congress stops funding of the "Reliable Replacement Warhead" ... so what does the Pentagon do? Conduct the research in Britain.
It's the natural result of the Atomic Weapons Establishment being managed by three private companies, two of which (Lockheed Martin, Jacobs Engineering) are American. As well, this kind of British-US joint research into nuclear warheads has been going on for years.
Kate Hudson, of CND: "Any work preparing the way for new warheads cuts right across the UK's commitment to disarm, which it signed up to in the nuclear non-proliferation treaty. That this work may be contributing to both future US and British warheads is nothing short of scandalous."
Also read: The Future of the UK’s Strategic Nuclear Deterrent: the Strategic Context (House of Commons Defence Committee, 2005/6)