Well, I have a Virgin pay-as-you-go but they take the money by DD every month and you get an itemised bill. Texts are usual price and 3p to other Virgin phones, calls are usualish rates but cheaper to other Virgin phones again. You don't have to get a new phone, just buy a SIM card, I think it's £10 including £5 of calls.
One of the best things about it is that you can use it abroad easily without topping up before you go, and you start out with a credit limit of £100 so I assume that if it got nicked that would be the limit of your liability.
This is their website although it didn't finish loading just now.