My BF has a Blackberry on contract. I think he paid for the phone outright and then got a £10 a month contract that gave him a good deal of internet, texts and minutes. He also has to pay another £5 for BB services, so it's about £15 a month in total. He's with T-Mobile. I wouldn't say they have the greatest coverage, but it works.
Dennis is right, though - you're best to go to Carphone Warehouse and see what sort of options you can get.