Here you go:
http://www.prepaidgsm.net/en/uk.html
This website is fantastic and will have all the operators, rates, and features available. I used the site recently before I traveled to Italy. I knew exactly what provider's SIM card to buy, where to get it, and how much it cost. I also knew what provider had the lowest data plan. I ended up getting a 3GB data plan for 5 Euros!! Verizon needs to learn a thing or two.