Well, as I say the cheapest ain't always the best. If you want great network coverage and are prepared to pay a higher tariff charge Vodafone is a must. O2 is exceptional for free text messaging if you get yourself a "genie sim card" which you'll be able to get online. T-Mobile not the greatest to be honest relatively expensive and Orange is great as it has three different plans on prepay. Virgin offers a good service as well (uses the same network as T-Mobile) but good prices. I would say the best thing to do is go directly to the websites of corresponding networks.