Some of the users on this fourm have spoken about giffgaff before: www;giffgaff.com

They are owned by o2 and run on the o2 network offering charges of: 4p a text and 8p a call, free giffgaf to giffgaff calls, Picture messages to giffgaff and other mobiles 16p, 0871, 0872,0873 numbers 20p,0843,0844,0845 numbers 20p and 0800 numbers FREE up untill May of this year they are offering free data. However this is about to change and they have relesed new prices from today:

£5 giffgaff bag - Unlimited Texts
£10 giffgaff bag - 100 minutes, unlimited browsing and texts in the UK for a month
£15 giffgaff bag - 300 minutes, unlimited browsing and texts in the UK for a month
£35 giffgaff bag - unlimited voice, text and browsing in the UK for a month

Note giffgaff have stated the following: don’t forget, unlimited is truly unlimited. There are no fair usage policies attached to these.