My MIL is going to the south of Brittany again this year, she has always driven further south in the pusuit of hot wether but was happy with the temp here.
What about the likes of keycamp or eurocamp, the ready set up tents are the cheapest option, or if you really want to keep the prices down take your own tent, MIL takes her caravan and has always been impressed by the quality of facilities. The campsites usually have pools, a shop and some kind of entertainment.