(U.K. only question, ta!) How did the different pronunciations of the letter 'u' come about in England?

What I mean by that is, why is it pronounced 'uh' down South and 'oo', (like the 'oo' in words such as 'look', 'book' and 'cook'), up North? Take the word 'London', for instance: down South, it's pronounced 'Lundon', (which, for the record's how I pronounce it, as a Southerner), whereas, on the other hand, when I first started to visit Liverpool, (MANY years ago now), I noticed that THEY pronounce it 'Loondon' up there, (as they also do, up here, [or, should I say, 'OOP here'], in Lincoln), and, I do believe, in MOST of the Midlands and Northern England). Like I said before, the 'uh' sound seems to turn into an 'oo' sound, (very similar to that in the middle of the word 'cook'), when uttered in the Midlands and up North, and that sounds very strange, (and, dare I say it, almost foreign to me!) Anyway, what I've always wondered is, how did them two VERY different pronunciations of the letter 'u' come about in England? I'd REALLY appreciate YOUR angles on this one, please, guys, 'cos I'm buggered if I bloody know! Much obliged! Peace and love! Be lucky! :)