If you live in the right area go for Blueyonder from Telewest, its the cheapest and fastest, also they usually throw in cheap telephone calls and a cable TV package.

However if your not then go with BT. Their currently offering 1 megabit line for £29.99 with 1 Gig a day bandwidth (download only) limit. Also they allow you to use internet connection sharing so you can have upto 10 computers connected through one line.

Also you can purchase a router and use that instead of the modem they give you free, and that means you don't have to leave one computer on all the time to service the others with the internet.

Hope that helps