Take a look at this handy guide, published by Ofcom, Consumer Focus and the Communications Consumer Panel. It’s been put together to help mobile users get the best mobile deal for their situation.
Although short, its full of useful tips and information on everything from checking network coverage, using a phone overseas, contract or pay as you go pros/cons and keeping your mobile number:
http://www.ofcom.org.uk/files/2010/03/mobiledeal_v2b.pdf