It's not offensive to refer to the Ninth Doctor as northern, because he's very much intended to be northern and played as such. Show him to anyone from England and the first thing most of them will say is "northern". It's why that rubbish joke about planets having a north is in Rose; he's strongly northern. And being so clearly regional is at odds with the Doctor as portrayed before; Sylvester McCoy is Scottish, but he didn't play the Doctor with a kilt and bagpipes, which is pretty much the equivalent of what Eccleston did with his leather jacket. Most people from the north of England aren't that strongly nothern either; he's very much a stereotype. Of course, his being northern isn't itself the reason I dislike the Ninth Doctor, but as I said, having a strongly regional identity is (in my view at least) at odds with the character of the Doctor.