touring in france in school holidays?

I have been touring France from mid August for the last 5 years - with ease. Our secret is to not prebook anything (other than the ferry and first night) and leave the rest to chance.  We have stayed in some amazing locations - all the more delightful becuase we stumbled across them. Interestingly we don't need a 4* star site, simply a stunning location, nice bar and clean loos.  Anyway this year we have to take our holiday from 20 July - right slap bang in the middle of French holidays. Do any other members have experience of being able to apply the 'wanderlust' approach during busier times?  I am keen not to book ahead as I want to be able to go with the weather.  Western France is my preferred choice. Grateful for any shared experience.  Sandra