I'm with
Metronet PAYGoDSL. The cost for a 512k link starts at £10 and goes up to a maximum of £23.99 depending on how you download in a month.
You have to buy your own modem, but the advantages are you get multiple static IP addresses, no speed throttling and a free back up 0845 dial-up account which you can use away from home. They also only have a 3 month contract period as opposed to a year, so you change or cancel at reasonably short notice.