Ah I see, never crossed my mind that was what you were meaning. Well, there are cultural differences between the two which can be pooly understood by people who haven't spent many years living and working in both countries. A simple example might be that you don't often hear Americans saying 'Please', but that's not being rude, they will wait until they thank you which they almost always do. Brits are now losing the ability to say thank you. Even within the US there are differences, New Yorkers are considered ruder than Californians - yet when a New Yorker moves to California they suddenly become all nice and obsequious
They also say that Americans can be friendly without being nice, Brits can be nice without being friendly. Or is it the other way round?
Still, I would wager that most Brits would say American face-to-face customer service is in general better than British customer service, and I would agree. So you claim a 100% bad experience with Americans, and 100% good experience with Brits, and I would say your experiences are unusual.
I'm still surprised this happened on Southwest. Had it been AA or BA I would be less so. Maybe it's a One World thing