Roger, I've just done the number crunching on distances, times etc as a future guide. Looking at late August out, late Sept back, starting in Hatherleigh mid Devon and aiming for Limueil east of Bergerac in The Dordogne.
The initial obvious choice of a Plymouth crossing doesn't really pan out when you look at the results. It would seem in future I'd be looking at an a very early morning departure from Hatherleigh, take the Newhaven Ferry and then drive for an hour or so after arriving in Dieppe.

Colin

Hatherleigh to Plymouth - 1hr 15mins + 38miles
Brittany Ferries Plymouth to Roscoff crossing - night out with cabin, midday back = £520 inc friends and family disc’
Roscoff to Limueil - N12 & D939 = 10hr 7mins + 446 miles and zero tolls

Hatherleigh to Poole - 2hr 40mins + 112 miles
Brittany Ferries - Poole to Cherbourg crossing - 08:30 out and 18:15 back = £423 inc friends and family disc’
Cherbourg to Limueil - D939 & N162 = 10h 38mins + 424 miles and zero tolls

Hatherleigh to Newhaven - 4hr 50mins + 220miles
DFDS - Newhaven to Dieppe crossing - 09:30 out and 12:00 back = £263 - 20% over 60’s disc = £210
Dieppe to Limueil - D565 & D704 = 9hr 49mins + 408 miles and zero tolls

Hatherleigh to Folkstone - 5hr 20mins + 270miles
Eurotunnel - Folkstone to Calais - Typically £520 return
Calais to Limueil - A20 & A16, via Dreux = 9hr 54mins + 528 miles and tolls at Euro 43 [non tolls = +2hr]

There is a useful campsite within 10 minutes of the Newhaven Ferry Terminal Colin. It's OK for one night (used to be 30 quid) and it'll save a very early morning drive for you.

http://www.buckleholidaypark.co.uk/about.htm

Also there are several good ACSI sites in and around Dieppe so you don't need to drive too far when you disembark in France.

I have made a note of these three for our return trip this summer. The first two are virtually next door to each other:-

4km, Camping De Trologot, Greve du Man, 29250/St. Pol-de-Leon - Free Wi-fi - ACSI-1077 - Euro 19
4km, Camping Ar Kleuger, Avenue de la Mer, 29250 / St. Pol-de-Léon ACSI-1076 - Free reception wi-fi - Euro 21
3km, Camping Paradis La Pointe de Roscoff - ACSI-1064 - Euro 17 - Basic site

Colin