Virgin Mobile offer some great pay-as-you-go packages combining texts, voice calls etc (alternatively you can just top-up your airtime in the normal way). They're constantly advertising special deals on phones and are often generous with the free airtime too when you sign up to them. I've been very pleased with them since I started using them about 6 months ago.

I started with vodafone about eight years ago but gave them up as a bad job and switched to Virgin. Their pay monthly contract has no line rental charges and you're only charged for the call cost. 15p for 1st five minutes in the day then drops to 5p a minute for the rest of the day. I defy any other company to give me a decent phone than sometimes costs less than a fiver a month to use!