I see they do the Iphone 5 on PAYG. Say I get it on O2 can I use any O2 sim in the phone or does it lock to that number?
The handset will be locked to the network you get it from, say if its vodafone, o2, orange etc. You will have to request an unlocked version of the handset from carphonewarehouse. However test the handset in store with other sims to be on the safe side. If your in doubt and want to purchase a handset unlocked on all networks, you will be best going direct to Apple: http://store.apple.com/uk/browse/home/shop_iphone/family/iphone5 or get one from an Apple store.