[Torchwatch]

France is a lot bigger than England, between the towns there are miles of beach, to get to these beaches you have to cycle or walk as often there is no road access. This means there are beaches with very few people about. If there's no one about there's no reason not to wear bikini briefs, swim thongs or even go naked. If you see someone coming they appear as dots in the distance and slowly grow larger as they come towards you. You'll have plenty of time to judge how to present yourself when they finally arrive.
The beach in front of a ville is usually packed with families if you want a peaceful and skimpy day just walk a couple of miles out of town. As you go you'll see more speedos and fewer shorts.
In Northern France there are good sandy beaches between Le Touquet (Paris Plage) and Berck sur Mere. I usually camp at Camping Ami Ami at Berck sur Mere, it's right behind the dunes. Naturist beach to the North and town beach to the South and other English never go there.
There's good sandy beaches on the West side of the Cherbourg Peninsula, in the North there are miles of dunes behind the beach with abandoned WW2 tanks and a half track lost there.
The area West of St Malo has beautiful sandy coves, it's a great cycle ride back to the ferry port in the early morning.
To go any further South needs a lot of driving (cycling or trains), Erdeven in the South of Brittany has great beaches and a cheap Camping Municipal at Kerhilio.
If you're camping and want good basic facilities look out for camping municipals.
In this area take a trip into Lorient, they have a WW2 German submarine base under a concrete dome, the Americans bombed it repeatedly and scratched the paint.
An even longer drive will take you to Bordeaux, There's more than 100 miles of beach here with villes 10 miles apart. My favourite is at Le Gurp which has a huge camping municipal. While Brittany was green the Bordeaux coast is beige, it gets really hot here, if camping take a tarp for your tent to get shade. This is classic get away from town and have a whole beach to yourself country. Get naked, wear a thong or something white that goes see through when wet and no one will mind.
It got so hot that my sat nav overheated on the motorway near Bordeaux and I had to buy a new one in a hypermarket. Take a good first aid kit, it may take an ambulance a hour to arrive, the distances are so great.

Many bigger French campsites have swimming pools, the no shorts rule still stands and you'll see men and boys in speedos, square cuts and jammers, a bikini brief would be accepted. If they have water slides the boys will tuck their speedos into thongs and slide down faster, no one cares though you may look twice first time you see it. Since it's easier to get changed in your own tent and just take a towel to the pool you'll find loads of people walking around the campsite in speedos with a towel over their shoulders, then if its warm they'll hang out on their pitches in their swimwear.

Where are you planning to go ?