This one just came into my head the other day, from the first NuWho episode:
Rose Tyler: If you're an alien, how come you sound like you're from the North?
The Doctor: Lots of planets have a North!
Christopher Eccleston has a strong Manchester accent, and it's a nice little indirect callback to the fact that, up until relatively recently in the grand scheme of things, if you wanted to work as an actor in the UK you had to lose your regional accent. Many people who you think of as having very typically plummy Received Pronunciation British accents didn't grow up speaking that way. Tom Baker, for example, is from Liverpool and Patrick Stewart is from Yorkshire. (And actually the Sixth Doctor, Colin Baker, was also from Manchester.)
For the next hour, everything in this post is strictly based on the available facts.