Originally Posted by stut

You're unlikely to be able to get a SIM that covers both countries particularly well - while UK SIMs will have European roaming, this may take some time to enable, and is not wonderful value, even if the prices are now capped by EU rules.

I'd suggest referring to the excellent resource for prepaid SIMs here:

http://www.flyertalk.com/forum/trave...m-threads.html