[Motel75]

Yep, TMO is the way to go (I'm using it right now to write this!) but there are a couple of caveats. 1) It uses a proxy server that severely compresses JPEG images; 2) there is evidently a list of restricted sites that you can't go to unless you register your credit card; 3) the terms and conditions specifically exclude laptop use, though it will work (VoIP and filesharing are also specifically excluded and probably blocked). The soft limit is calculated on a monthly basis, but please don't abuse it for the sake of other users (like, uh, me).

After your 10 days of free browsing are up, you can purchase a five-day pass for 2.50 that offers the same thing (half the usual price).

Vodafone also has a one-pound cap, but starts charging again once your use exceeds 15 megabytes (at, I think, 3 pounds per MB).

Virgin rips you off -- 5 pounds per MB.