Tesco Mobile is actually 02. (Tesco don't have their own transmitter network).
02 buys coverage in France from Bouygues, Orange and SFR.
However Bouygues uses the 1800 Mhz band, so you'd need a dual-band phone to connect with with their network. If your French is good enough, you might be able to find a coverage map here:
http://www.bouyguestelecom.fr/
Orange and SFR are both available to users of any GSM handset, although SFR doesn't fully cover some parts of the country. Website here:
http://www.sfr.fr/
and here:
http://www.orange.fr/
Chris