If you are thinking of Pay Monthly and sim only .. have a look at ..
http://giffgaff.com/.. runs on the o2 network.
Choose the minutes and texts that suit you, and browsing is unlimited.
Personally I would'nt get involved in a longer term contract, as there are quite a few sim only deals available, with just a short 30 day rolling contract,
this leaves you free to jump ship when you spot the next best deal.
Whichever way you decide to go, make sure you check with a cashback site first, as there are plenty of deals around where you can save £££s on a new contract or phone.