Kit Harington is from Worcestershire, IMDB tells me.
The accent thing is kind of strange, isn't it? Westeros is a continent, right - which should make 'The North' equivalent to Scandinavia, or even Canada. So why is it that everyone from north of some arbitrary line talks in a generic Tyke accent? If people from Winterfell sound Yorkshireish, shouldn't the Wildings sound Scottish? Or at least Geordie? (Ha, that would be wicked, wouldn't it...)
Maybe the directors considered this but then thought it would be lost on American audiences, I dunno.
With Joffrey, the basic principle seems to be that whenever you think he's about to do something horrible, think of something worse than that and that's probably what he's going to do.