I know what you mean about the variation in how people speak. I lived in Glasgow for a few years and there's quite a range, just like there is my home town. My ex is from a WC background, comes from a part of the country where there's a very distinctive local accent. He doesn't have it - very much wc made good.To be fair I sound a lot posher than my background would suggest (given that I think I’m common as muck). As does my mum. People would probably be able to place my accent if they met me but there are people in my area who talk through their nose like they are a character from chewing the fat. Accents can vary dramatically even in a few miles.
Just a theory of mine, but I think how you are educated (private school, comprehensive, level of education and so on) has an impact on how you speak.
Am conscious of starting to waffle on irrelevantly so I'll leave things there!