I think T-Mobile might do them but Vodafone definitely does and I even wrote a big article on it here, which should come in handy:
http://www.ispreview.co.uk/articles/09_how_to_use_mobile_broadband/
1GB top-up's cost £15 but don't expire, the initial pack costs £39 for the modem and 1GB included. Don't forget to review whoever you pick as we're a bit short of Mobile Broadband feedback.
http://www.ispreview.co.uk/review/categories/7.html