I have noticed, however, particularly in London, that accent seems less a marker of geography than it is of social status. The 'cut glass' accent is widespread, but rare. Estuary English seems to be the choice of the media classes. Sarf-east Lunnon is everywhere amongst the lower middle class, and so on. The working class, I suspect, is more easily geographically identifiable, but only because they are less likely to be mobile.

The whole subject is as you say, fascinating.