I don't know, I mean Andrew Lincoln and Stephen Moyer are both doing regional American accents, with the former portraying someone from Georgia and the latter Louisiana. In fact, I have a suspicion that the Deep South's accents are easier for British actors to emulate, but I need more evidence before I'm sold on the theory.
British newscasters tend to have the "Middle English" so-called accent. It's something of a socio-economic as well as regional thing, with people from the south and east of England and usually of middle class upbringing and education most likely to speak it. But, for instance, my accent is closer to that than the region I lived in, as were a good number of my friends.
Also, the BBC has come under a bit of criticism for its hiring practices based on accent, so nowadays you do tend to hear more Scottish accents (usually from Edinburgh/Fife), the occasional Cardiff Welsh accent and Essex. Not so much Newcastle, Birmingham or Glasgow. Middle English still predominates, but outside of the international newscasts is becoming less common.
