"speaks with a 'Mummerzet' accent, a county that doesn't appear on any real map of England"

No but the said country is accessed through a London railway platform and most of characters speak in English accents and his is of course supposed to be English. The point was though that Coltrane doesn't speak in his natural accent. Like Emma Thomson doesn't. They are acting. So Thomson shouldn't be blamed for cultural colonisation (as is suggested in the blog) when all she is doing is acting - and she has a Scottish mother anyway! As I said you can't define other people for them. My wife was born and brought up in Norfolk but has a Scottish father. She's lived back here for 25 years now but even before she came up she regarded herself as Scottish! Her brother sees himself as thoroughly English. People will decide for themselves which part of their heritage they associate most closely with - whatever their accents are.