If you are crossing to France via SAM and MP check if the French shoot down zone on the Cherbourg peninsula is still in operation.
The hills south of Biarritz are actually mountains, part of the Pyrenees. They seem to exacerbate the formation of impressive CB's. Biarritz used to have a good met office with radar screens showing the CB activity.
Is your club trip routing north of the mountains in France, or south in Spain? The terrain south is pretty inhospitable if an engine stops.
If going north, winds along the Carcassonne to Perpignan valley can be strong and conditions turbulent.Read the TAF's carefully.