There's some fairly well priced SIM only deals to be had out there. One of the cheaper and most hassle free / least commitment ones is with GiffGaff - worth checking their site for current pricing and it's just a rolling monthly contract. They run on the O2 network.
I've been with Three now for a few years though and been pretty happy with them too. If you don't use much data when out and about you should easily find something for about £10 a month.