GiffGaff (www.giffgaff.com) is a mobile phone provider in the UK which has worked for us as an easy and cheap way to use our mobile phones here. They piggyback off the O2 network so have good coverage.
As long as you have a UK address they will post a SIM to you for free. Pop it in your phone and go on-line to create an account and buy a monthly 'GoodyBag'.
For £12 we have 500 UK Calling minutes, unlimited texts and 2GB of data. £15 gives you twice that. If you run out, simply buy another GoodyBag, it couldn't be simpler (no term contracts or term; all you need is an unlocked phone).
The only hassle I had was paying by Credit Card. It wouldn't accept my Visa or MasterCard Credit Cards because their statement address did not match our friend's UK address (where the SIM was sent). This was resolved by using my Airpoints OneSmart MasterCard (which is a Debit Card). I'm glad we brought multiple bits of plastic with us!
Our mobile phone numbers here are:
Richard: +44 (07521) 238 722
Miriam: +44 (07521) 242 867