From my experience summer is to be avoided as surf is inconsistent and the crowds are mental. On the plus side there are plenty of beautiful French women on the beaches. Autumn to spring will provide the biggest swells, however Sept/Oct is when your most likely to get good weather the water is still warm and this is hurricane season in the Atlantic so a good chance of solid ground swell. This applies to the Southwest coast but there is a lot of varied coast in France. My advise would be to invest in a copy of The Storm Rider Guide Europe.
at this stage planning Biarritz in Sept. can you recommend somewhere to stay there ?, car hire ? all round costs and board hire ie dont want to lug boards through Europe - I would if dedicated surf trip (if good boards available) ? and Im on a budget too
I got some good waves in San Sabastian, Northern Spain in September couple of years ago. Hired a board on the beach. Most of the surf shops in that area have board hire too. Can be worth driving along the coast there are a lot of different places to surf along the coastline. I had a hire car and found many different waves from Southern France to Portugal