Whatever you do avoid O2 as a provider - rubbish customer service and just disconnected my phone with £30 credit on it. 'Your sim card is more than 3 years old' they said.
I am with O2 Mosaic, have been since they were BT Cellnet (back in the 1800s). I'm pay monthly and have never had any bother with them. In fact I usually manage to whittle something extra out of them when I renew my contract. ;-)
Why can I only get phones that have 4G!!! We don't even have 3G up here!! And this nonsense about having a monthly tariff for the phone and one for the actual usage!