Birmingham UK-South of the Bull Ring around Digbeth Coach Station. The area's undergoing a bit of a revival now, like many areas in Central Brum, so alot of the riffraff has moved on, but I've never been so happy to run into a cop than in this area once. If you're there, and things are looking a bit dodgy, find the White Swan or the Anchor. They're safe havens. The Paddys at The Swan, in particular, will treat you well, AND you'll get the best pint of Guinness that side of Dublin.
Anywhere in the Midlands can be a bit dodgy really. Walsall, Woverhampton, Coventry, Dudley, West Bromwich, etc. all can be pretty violent for no apparent reason. When I lived in the area in the mid-90s the little bastards only carried knives and icepicks, but now I hear that guns are appearing more and more.
I have also walked through, and drank in pubs, in what were apparently pretty rough areas of South London, Edinburgh, Stirling, Manchester, and Nottingham, but being an innocent, little Canadian boy(and after more than a couple of pints) I didn't know any better. Having a Canadian accent helps get you out of A LOT of potential negative encounters in the UK. Someone's always got a brother/sister/cousin in Toronto or Vancouver.