A new option I have just become aware of is a SIM Only version of Mobile Broadband. This has some of the benefits of Pay as you Go as well as some of the benefits of contract. As it is SIM Only there is just a 30 day rolling contract so you can cancel the contract at any time, giving 30 days notice. But, as with a contract you get your bill each month to pay, so it automatically renews each month with no intervention.
I only know of one company at the moment that offers SIM Only for Mobile Broadband, which is O2. You get it with a USB Modem which costs £99.99 and the monthly line rental is £20.00.