Dappers advice is the best (check the coverage data) currently tmobile is working the best for me (and its cheap) there are loads of web deals out there that are usually better than the shop deals, as in a lot of web deals give unlocked phones rather than branded and locked ones.
one company i have used recently is
www.mobilephonesdirect.com cheap prices and good packages on the latest greatest handsets.
p.s. in april orange and t-mobile merge, so "if" they share their networks then it should be even greater coverage