I can't really help you, but I just thought I would let you know that you are not alone in your quest. We have just booked to go to France around these regions on 14th August (why don't you book then and join us), so I will be watching any replies to your post with great interest. This is our first time going to France and I have been told that there are many places where you can just turn up.
We never go to France without our Alan Rogers site guide, just look up campsites in the area you wish to stay and ring ahead to see if there are any vacancies. That said, we have often just turned up without booking and always managed to find somewhere. If going on spec, just don't leave it too late in the day when you are already tired as, if you don't get fixed up at the first campsite you try, things could get a bit worrying.
End of July through August is the height of the holiday season in France hence the most busiest time. All popular sites in Brittany will have been booked up well in advance. I'm not saying you won't get on but the chances are a bit slim. You will have more chance on the municipals as long as you turn up early, don't leave it late incase you've got to move on elsewhere. I've stayed on municipals many times and some are quite superb, but you don't have things like swimming pools or restuarants, most of the towns have one. Buy The Michelin Guide to Camping and Caravanning for France. It's in French but gives a large selection of both commercial and municipal sites and if fairly easy to understand. Your choice of areas is much to vast to recommend but your best bet will be to stay slightly inland if not booking.
harryB
Don't forget to leave a review of the French and other European campsites you have visited!
If you want the little sites and you've not got kids with you... just go. There are a lot of villages with little campsites (usually Municipal Campsites) and if you are content to pitch your tent and wander into the village in search of some grub you should be fine. As others have said, don't leave it too late ... 4:30-ish is about right in case you need to look in two or 3 villages for a decent site. Little village brasseries often close around 8:30 or 9-ish ... we've fallen foul of that before
On the other hand, if you've got teenagers, need flumes and a site that's directly on the beach... you'll need to book. We've been to sites like that a couple of times and not booked but you get a max of 3 nights or so on one pitch and have to move pitches if you want to stay on the site.
You have to book to be sure of a pitch. Camping is definately getting more popular every year. Used to book sites around March. Now its October or November.