London. Logistics are simple, time zones work, and they have large NFL fanbases. None of which is true of the UK.

If you go to the UK, I think you need an entire 4-team division in Europe; otherwise, the logistics are just too complicated on a full-season basis. And even then, they’re pretty complicated. Any team in an isolated London franchise would have to make 8 transatlantic round-trip flights per season, across 6-9 time zones each time — their record would be atrocious just because of jet lag alone.