There are funny little pockets of accents just a few miles apart. in Ashington (Northumberland) the folk speak Pitmatic, a rare accent or pronunciation only found there. And Scouse (Liverpool) stops abruptly when you leave the city, becoming Lancashire (quite different) In Norwich, people speak a nasal 'Narridge' with quite broad vowels. But move two miles out and the lovely rounded country Norfolk accent takes over. It's always fascinated me how UK accents change as you move east, west, north or south just a tad.
Regarding 'Mid-Atlantic', I don't think it means people who actually live in the middle of the ocean. It's merely a way of speaking which tries to find a middle course between American and British. I first heard it on radio programmes, when disc-jockeys on Radio Caroline spoke like that. It doesn't actually mimic either accent, it's a sort of mongrel talk!