Both cities have high numbers of ex-pats from the mainland. Logistically, it's probably best for West Coast teams to fly there, London is really dumb for the Rams now. So you're looking at Rams / Seahawks, or Rams / Raiders (not gonna happen) or Rams Chargers - the NFC West / AFC west timing would have to work out for that to happen and the Chargers don't have a big fan base outside of their area.
Unfortunately this game makes sense for the league, wonder if it was part of the moving bargain Stan struck.