”I cannot recall hearing "he's a regular guy" ever being used in the UK. I don't think anyone here would imagine it as an insult.”

Me neither, Raggy. It’s simply not a part of the UK lexicon.

The one that grates with me is, “I couldn’t care less” (UK) v. “I could care less” (US). The US version clearly doesn’t mean what they think it means!

And the strangest expression I’ve heard from an American in recent times was ‘deplane’, meaning to disembark from an aircraft.