Emmerson.
I would be looking for ferry deals now if I were you.
Some of the larger French sites close in September after the Brits head home from the school hols I understand so it may be worth checking if chosen sites stay open or picking sites that stay open.
We have done two part holidays in the past, from West Coast and then down to the Dordogne, the Med,Italy, Switzerland. Kids when younger made friends on the west coast so we did annual trips so they met their holiday friends. Thats how my kids speak French!
Depends how much you want to see, in recent times I get booking deals with P&O going to France. For sites I just call a day or so before we go and have always found space available, but I would not reccomend trying that when the French are starting their annual holiday shut down but you will be clear of that.
Travel is generally much easier than here so you should have no problems getting to see what you plan.