Hi Michael
My recommendation would be Camping Du Letty in Benodet. A super beachside site which is my family's favourite site in the whole of Europe!!! I have put full reviews on the UK Campsite and Touring and tenting websites if you want to read more.
I have done the journey from Cherbourg. The AA website routeplanner will do you a route plan to the actual campsite if you input Camping Du Letty as the destination.
Have a super time in Brittany wherever you end up.
Regards
Tim