The third carrier to sell the iPhone in the UK is here. Tesco Mobile, which operates thanks to an MVNO contract with O2, is introducing today the 3G and 3GS to their offerings. As we saw with the Orange tariffs, Tesco is not going to break the bank to make this a successful launch. Instead of a 18 minimum monthly contract, Tesco is going for only twelve. This fact alone is going to attract a lot of current iPhone users that are currently locked in a long contract and can't upgrade their model.
Tariffs are very competitive and specially cheap for the old generation 3Gs. See the full tariffs at Tesco Mobile