I haven't read the entire thread. Couldn't it be possible there's a Boomers type faction in the UK, with access to functioning aircraft? After all it was never said that only the Enclave and NCR have access to aircraft. Or there could be remnants of the UK's version of the CIA, which still has functioning aircraft.
Ok, I just read that the UK was so busy bickering amongst themselves. Maybe they didn't have time to prepare like the US did, with the Enclave and Vault systems.
Edit: They could have created boats/ships that can sail the vast ocean? After all boats/ships have been around for thousands of years. If the Vikings can do it, why can't the wastelanders?
I can see Tenpenny having a ship constructed from various parts, then assembling a crew to set sail to America.
Edit 2: The real question is: What is so terrible about the UK, that one would risk sailing to another continent?