If you just want to stick with a UK SIM, try GiffGaff. It's 10p/min for calls, 6p for texts and 20p for the first 20MB data per day plus 20p per MB thereafter, so much cheaper than O2. For France, it's:

Dialling within EU and select countries plus calls to voicemail (charged in per second increments after first 30 seconds) 24p per minute
Dialling outside this price band £1 per minute
Receiving calls (charged in per second increments) 7p per minute
Sending texts 8p per text
Receiving texts Free
Picture messages (MMS) to a UK mobile 16p per MMS
Picture messages (MMS) to any other mobile 24p per MMS
Data Roaming* 45p per MB