>London - accent seems less a marker of geography than it is of social status

The difference with London and say, Nottingham is that very few people you meet in London originate there. The taxi drivers and typists come in every day from Essex or Kent or whichever part of the estuary is closer. The media types probably come from Cumbria, Edinburgh or Manchester. If you are in a famous London teaching hospital many of the staff will be from the West Indies or India. It's a great big melting pot.

I'm sure the same would be true of other world cities. I wonder how many of the people in top jobs in New York were born there?

Regional accents in the UK do tend to be very local. Manchester, where I went to school, had a range of accents which seemed to change every five miles or so, add a few Asian and Caribbean variations and the range was huge.