Hi Grenville.I think you will find that Brittany Ferries will not be running from late November until spring, so you will have to check your travel times for availability.
We've done this trip a number of times, both using the short sea crossings and the Bilbao crossing.
One major problem with France at that time of year is the dearth of sites open for overnight stops.
The route from Blbao/Santander to the south is quite easy, Burgos, Madrid, Granada.
I drive a 3lt 4X4 but even with this it's difficult to justify the Santander crossing on cost alone. I can say though that the Santander route is a much quicker and more relaxed way to reach the fine weather.
One tip on finance, if you haven't already done so open a Nationwide Flex account and take out a Nationwide credit card.
Using the flex account to get your money from an atm there are no commission charges, similarly the credit card is commission free within the EU.