AH! Lovely! A discussion of various English accents from all over. I've started to be really interested in accents since a few months ago. Of course, I have a small problem, which is that it's really hard for me to hear subtle differences in pronunciation.
That Little Britain clip was a pretty good immigration Texan accent, but I don't think it was spot on. There was something a bit off about it, though I couldn't say what. I'm tempted to forward that clip on to my Texan friend and hear what he has to say about it, except I'm not sure it's quite "safe for work" enough for where he works.
Something I've noticed for me is that the more homogeneous everyone around you sounds, the harder it is to identify how other people sound different. I grew up in Southern California and I'm now living near San Fransisco, so to me, pretty much everyone I've ever talked to has spoken "Hollywood" American and everything on TV and most movies use the same accent. I think most of the other accents I've heard in media have probably been bad fakes like Dick van Dyke's "Cockney" in Mary Poppins. I just thought that's how Brits spoke for the longest time. Heh.
It's just interesting to me that some people can so easily adopt a non-native dialect and do it well, while some of us are only capable of making fools of ourselves.
I'm pretty sure I'm in the later camp.