The one Americanism that really irritates me is not a matter of pronunciation (many Americans pronounce words more clearly than we Anglos do, which is fine) it's saying nthe exact opposite of what is meant: "I could care less" when what is meant is "I could NOT care less..."
As many English words are derived from French, they get annoyed with us for changing their pronounciations and we get annoyed with the Americans for changing ours and when anyone uses correct French, they are called pretentious! It's a minefield
On the subject of transatlantic accents, I have just had an email from DDiL (American) to say DGD (just two) who has lived in USA since she was three months old, insists on pronouncing tomato the British way. When her mum uses the American 'tomayto' she says, " That's wrong mummy, it's tomato." They emigrated nearly two years ago and there is no sign off DS1 losing his accent, English with a mixture of Yorkshire (me and DH) and Cornwall & Devon (upbringing/education).
I can no longer remember which is UK pronunciation, and which is US. I think I've been saying skedule for some time now. And first my kids mocked me for saying zebra so I changed to zeebra. Now they're laughing at me for saying that and I've got to remember to go back to zebra! Can't keep up with it.
Within the last few minutes I have heard "furoar" and one of todays horse races referred to (by whichever sports correspondent is on duty today) as the "seezer witch" It's actually the "Cesarewitch"