there is (AFAIK) no prepay/one month contract giving data roaming throughout Europe on one SIM at an economic price (I've seen some but the data costs are way higher thqan locally purchased SIM's).

The Tre Italia (Three/Hutchieson Whompoa) deal that debernardis' mentioned is really good - particularly in your case as the same data bundle is useable in both Italy & UK. .I use this frequently, it's reliable, easy to set up/cancel, lasts for a calendar month (or part thereof) and is great value. I'd reccomend this if you are

a) going to Italy before UK (note - you CAN'T buy a 3 UK SIM and use it's data bundle in Italy ... it used to work, but not any more)

b) going to be mainly in urban areas (where 3's coverage is generally very good) - in more rural areas, coverage gets patchy (as they are a pure 3G network) and you'll roam onto anothr operator's network (at additional cost) if you don't prevent this in your SIM settings.

I can't reccomend anything in France but I'm sure you'll get a local prepay card there OK. The
"one card for all" Europe wide solution is a lovely idea (the EU should impose it, at a sensible price!) but it doesn't really exist yet for folks who are not having their phone/data bills paid by a company.