If you’re not already with O2 or aren’t eligible for the above options, you can use the iPhone on a Pay & Go basis. You just need to get a Pay & Go iPhone SIM card from any O2 store. The iPhone SIM card is set to the Favourite Place tariff, so gives you unlimited calls to any UK landline or O2 mobile from your chosen postcode when you top-up £30 or more a month. If you don’t like the sound of this tariff, you’ll be able to switch to one of our other Pay & Go tariffs.
With the above service, there’s a “web bolt-on” that’s about 15.00USD a month.
Also, there’s this.