Mid-Atlantic accent

One of the following a) A British person who has lived in North america for long enough to start speaking with an accent that is on first sounding american (although there is a slight hint that they are British when thier accent slips), but when in the company of his/her fellow countryman/woman will start to speak properly. Usually, these people did not have strong accents in the first place. b) A north american who has emigrated to the UK and has started to sound like the local people, but still has a slight twang.