Like Ian has suggested, you may be better off going for a contract on a free anytime minutes + texts scheme - many of the latest offers give you monthly rental cashbacks and you will get a new phone as well.
Another important aspect is selecting the best service reception for the area you live/work etc, or where you spend most of your time - I'm with Orange mainly because its the only network which gives a good signal where I live, although they seem to be pretty good with their customer service as well.
I also have a work mobile on Vodaphone as well, and that seems to get good coverage in most areas too.
O2 seem to be doing the most agressive deals at the moment, but just check around from sites such as Ian suggested and find the best/cheapest for your needs