I've got an Orange PAYG sim in the cupboard with a bolt on called Out Here that orange sold for £15 for a very limited time, basically it gives you 5 free texts a day so long as you have credit registered on the phone. So if you don't call anyone, don't send more than 5 txts a day then it will never cost you a penny.

Very occasionally these sims pop up on ebay, and they sell for up to £100 sometimes!