I believe she lived in London (Islington??) for 4 years, ages 10 through 14... so didn't really 'grow up' there. She moved about a bit as a kid though, a US forces family as I recall, so probably her accent moved around equally. I believe she got the piss taken out of her when the family finally settled back in the US. That kind of thing tends to shape you - I know, as a little pommie kid growing up in NZ, I got used to being 'different'.
Anyways, her accent in 'The Fall' (and it's as much vocal mannerisms as accent) is definately not north London! It's a kind of clipped, upper class-ish slow and precise delivery that defies exact localisation. I find it hugely irritating.