What's the best mobile phone or simcard for a visitor to the UK / EU to use for a few months?

I use Three.co.uk - get a SIM card in the airport, and for a £15 top-up (on their website or via a voucher at an ATM or store) you can get unlimited data, 300 minutes, 3000 texts. They have other options that might be better suited to your needs for less. The best part is that when you go to certain countries covered in their "Feel at home" feature, you get to use your allowances as if you were in the UK - basically free roaming. They cover 11 countries now (including Austria, Italy, Ireland, Denmark, and Sweden), and are adding more from 1 July. Otherwise roaming data rates can be a bit steep, (45p/MB) such that I would avoid using data in all but emergency situations - roaming calls/texts are reasonable. I think they offer data buckets that can make roaming data cheaper in such situations.

tricky aspect seek at yahoo or google that may help

Thanks Daniel, that's very helpful.