OK, Musick - I'm definitely prepared to agree vehemently with you, given your gloss to what you first said.
It'll be interesting to find out what happens to CapK s god-daughter when she grows up. My two were born and bred in Northampton, and had very broad Northamptonian accents (which would be significantly different from CapK's g.d. if she lives, as I suspect, in Wellingboro - that's a whole twelve miles away!!) My wife and I speak a London debasement of RP (yes - it is the same sort of thing as SE) - guess how my kids speak now, in their early thirties? Yep, you're right - not a trace of N'thampton, me dook, but a flattish Estuary English.
My daughter did live in London for about six years from age eighteen, but my son never has done - and they both speak the same.