If you don't make many calls or texts' then try Ovivo.com, they have a sim only deal 150mins, 250 texts and 500mb of data. You pay a one off £15 for the sim and n o t h i n g else.
You get the £15 in credit too! only caveat is that they display an advert when you access the internet, but you just scroll down and select ok, and surf away.