Be very careful doing that, I have been hearing about people installing keystroke leggers and recording passwords at those places. I would double check with regulars and the owners about security before inputing your information, just incase.
Take your PC with you. The net cafes I've seen are waaay to expensive.
You'll probably spend less money using a temp ISP, like freeserve or BT anytime? Whatever they're called, just find a phoneline, plug in your modem, enter phonenumber/login and you'll be charged at localrate phonecall(0845).
Novamir proposed a London-meet in the following link and mentioned 'Nethouse' cafe near Baker Street (ugh, West London); I seem to recall he gave directions to the place as well. So that may be a start.