The best deal for europe-wide roaming is the Wind MEGA No Limit card, which I think is available in various pre-pad tarrifs, but all are very cheap. Unfortunately I don't think they are sold any more except but there are a few on ebay, search for "wind sim" and there are a few available, albeit listed in german and italian.

For the UK only the best is Orange pay-as-you-go but with the "Orange World add-on" which gives unlimited access for £1 a day - it isn't published widely but it gives unmetered GPRS internet access too.

Gareth