Head due south- say Portsmouth to get the LD lines ferry to LE Havre, or Brittany Ferries to Caen (expensive!). Or fly into La Rochelle or Nantes and hire a car but of course you couldn't really fly with all your camping gear! To plan the road journey try the website 'via Michelin'. Bear in mind France is a lot bigger than the UK!
We used to drive from Ayrshire to The vendee, Brittany and Biarritz - long drive but friends of mine are en route to tuscany as we speak - from Irvine!!! They ahve broken up their journey with a 3 night break in Switzerland. You could do the overnight ferry jaunt which is fanb (Portsmouth - St malo !!!) but it is very expensive. Or you could plan in overnight stops. Your drive to Dover will be the same as your drive to any southern port - depends on where you wnat to go on the other side. About 7 hours from calais to the Vendee.
2 years ago in St Jen de Monts we met an Irish family who flew into Nantes and hired a car. Said it was best thing ever and they manahed without the half ton of luggage we had. I'm flying with 2 DC's to a campsite in Italy thos year and I'm sure we'll be fine without the acr load of luggage. You can always buy what you need over there. Fly drive is a good option. Depends on how well your family travel.
We flew from Manchester to Nantes this year (and last) and hired a car from the airport. Then 2 hours to La G. We managed fine with the luggage - took our own carseats for the children because they cost loads to hire. We stayed in a mobile though, not in a tent. And DH is the world's ponciest chef on holiday, so we were probably the only family who carefully packed their own rhubarb and loose leaf gelatine!
Thanks for all your help. Children managed the trip to Cornwall okay (DH didn't manage as well...he doesn't do selective deafness as well as me!), so I think they could manage ok. Just think it would be easier to drive but I could be wrong. Now I just need to convince hubby that Mnetters are great at picking holiday destinations!!
For who flies where (budget airlines mostly) look at flightmapping - click on country then mouse over destinations & it will flash up nice red lines to show the routes - click on destination & you get details of airlines