Giffgaff is a mobile phone company run by its members. Get rewarded for helping other members or recruiting new ones. Even better, calls and texts to other members are completely free - see the site for full details.
The pay as you go rates are great - compare them to your current provider to see. You can buy a package of calls, texts and data - and change it whenever you want.
For an idea of prices - for £10 a month (buy whenever you need, not necessarily every month) you get
Vodafone is the world's largest mobile phone network with a massive global brand and presence. In terms of coverage it is number 1 in the UK and has pioneered the development of 3G products and services.