If you're going for Pay as You Go (which sounds most reasonable), it may be worth looking at Orange. My parents have SIM cards in the UK that IIRC top themselves up automatically by direct debit when they reach a certain trigger level. That way, you don't end up running out of credit. They also have international roaming on those SIMs (I know my Dad has used his in Thailand and France).

I don't know the full details and don't know how the rates compare with other networks but they find it a low-hassle way of getting low-usage phones.