Birmingham! Birmingham is gritty, post-industrial, urban jungle but very edgy, ethnically diverse, with some great (cheap) restaurants. If you have only seen London and the usual tourist sites around the UK (York, Oxford etc) be prepared for a massive surprise. Birmingham is partly gentrified (old canalside area converted into apartments, bars and pubs), and partly run down - walking around the old abandoned industrial paths that some of the canals lead to is truly fascinating. There is an old Irish Quarter, the old Jewellery Quarter (where they still have 100s of jewerlly workshops), the Chinese quarter (with awesome and cheap authentic Chinese and Vientamese restaurants like Cafe Soya), an artists' quarter in a converted former custard factory (called The Custard Factory)...
It has the best Indian restaurants in the country, and an incredible nightlife, great old pubs etc. It's manic, a bit random, it's not pretty, but its fascinating and it's the 'real England' in many ways :)