I used to teach windsurfing in Quiberon... the traffic is bad (basically all summer) and if your stuck on the presquile it could take you forever to move to a different location..gusty offshores on East side of peninsula.. and can be heavy onshore at Penthievre (near the Spit on West coast)... for better conditions head up the coast...
la Torche is well documented (therefore busy) ... but try the other end of the same bay at Penhors if the wind has any NW or S in it..much quieter and more space than La Torche. A little further west you get some good blasting conditions closer to Audierne..(Plage de Kersiny / Mesperleuc are good launch spots in NW / W) The reef off Audierne can provide some offshore fun & games 'La Gammelle'..(just follow the moaning of the depressing cardinal mark)
If you move up to the next peninsula (Crozon / Morgat) Goulien is awsome in SW.
Check out Aprils Boards mag for the article on Brittany.. all top sailing spots, similar to North Cornwall but with more variation for different wind directions so long as you're prepared to move about up to an hour... there is somewhere for all conditions..
check out this site for good spot info in france
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www.windlords.com]
Bonne chance!