BM,
I think you can only get an iphone through O2. However, a couple of my mates got one and it broke within the first two weeks, so they got a different one, so if you do, get insurance with it. In terms of money I think O2 are the cheapest in the UK, if you don't want a new phone look at the simplicity tariffs, I dont think you will beat the amount of txts and peak time minutes for the money anywhere.
Shang