Are there any offers due on the ferries to France for the last week in August or is that it for this year? The cheapest I can find is £154 return for car + FC which is Dover-Calais (or Dunkirk). I'm kicking myself for missing the DFDS offer at the end of last year. We can leave it till about a week before if the prices drop.
Don't forget to leave a review of the French and other European campsites you have visited!
Prices are unlikely to fall now so I would book asap. If you are booking in bank hol week it will be more expensive. Travelling early hours will reduce price.