Hi Jim,
For data I'd suggest the Three 3GB/90 day SIM card - costs around £20 on Amazon - gives you 3GB in the UK and the same in France under the data feel-at-home option.
http://www.three.co.uk/discover/phones/feel_at_home
This is for data, but it will work in both countries.
For voice I'd probably go for
http://www.togglemobile.co.uk/home/en - cheap calls in the UK and back to the US (3 pence a minute) and if you register a 30 day local number in the US and France, you get a US number for people to call you on for free, as well as cheap calls in France.
Hope this assists... Steve

[size=2]Steve Gold
PO Box 1014, Sheffield S10 5YG, UK
Home mobie: Telefonica O2
Other UK mobiles: 3, Vodafone, Virgin
Foreign SIMs: Toggle (multi); Germany (Fonic); Poland (Orange PL);
Skype: stevewgold