Do you want a contract or pay as you go?
Approach the different mobile networks (website or phone them) and ask what deals they have on offer at the moment. You can get SIM only contracts from all the major providers.
If you fancy the best mobile deal available at the moment, in fact this is the best one I have ever seen... with totaly unlimited calls, texts and data.
Take a look at this one. If you register with 'Topcashback' and take on the 12 month contract by purchasing the deal through their website. You will only pay £7.59 per month, for 12 months. The £101 cashback gives you the deal for a total cost of £91 over the 12 months.