Assuming value for money is your gripe for being "stuck on pay as you go" - then I think things have moved on. Many pay as you go deals are based around regular top-ups and give you contract like value for money. E.g. have a look at the orange link I gave above, or the t-mobile offers:
http://www.t-mobile.co.uk/shop/mobil...pay-as-you-go/
If it's all too confusing online the just use any UK sim you have for the first day or two and then wander into a carphonewarehouse or similar and get advice.