I've been browsing Ebay for a mobile phone for my stepdaughter, and I've noticed several PAYG sim cards for sale.
Please, DON'T buy these!! I know for a fact that Orange sells their SIM cards for £1 including postage and packaging, with £1 free credit - so they're basically free! I also know that Vodafone send theirs out for free, again with credit. I know because I've ordered them. I recently ordered some from O2 - again, free. The networks limit the amount of cards you can order to one house - but I think the maximum is 5, which should be plenty.
You just go to the networks websites, if it's not glaringly obvious where they're selling the SIM cards, just type 'sim card' in the search box - et voila!
SIM card sellers - I apologise, but us buyers hate finding out later that we could have bought something a lot, lot cheaper.
I hope this helps at least one person - especially at Christmas.