I also use Cherboug or St Malo each year for that area. Frankly, december is not the best time to see France - factor that part into the return trip.
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There will be no "best" route as it will depend upon the weather and your ferry times.
If going overnight, the cherbourg to Jauny Clan (Poitiers) or Pons - north of Bordeaux will be about right but near 300 miles than 200. if you don't mind paying the tolls the autoroutes make for very very easy driving.
Next night - St jean de Luz almost on the French / Spanish border. Several sites but La Rouletta just off autoroute junction is excellent and very popular. Google the name and get the details.
Next day comes the choice. For Valencia area, go into spain - about 15 miles in follow signs Pamplona and Zaragoza (all 4 lane but with tolls round Pamplona, Before you get to Zaragoza follow turns to Magallon and go to Sabinan aka Savinan which is in the book under Calatayud.
Well run site, high altitude so cold but hard and clean and quiet.
Then leave and go a few miles south towards Calatayud, then to Darocca and rejoin the autoroute (autopista in spain) to Teruel and then to Sagunto just north of Valencia. Sites in Valencia to the south and in Moncofa to the north. Then the east coast of Spain is at your disposal.
Can be a long way between filling stations so start looking to top up at half tank not any less. Autopista Darocca to Valencia is very good but almost empty - driving easy, boredom the biggest problem. Quite possible to average 50 mph without stress. very few pull-offs except at service stations.
The other choice - if you decide to skip Valencia area and go directly to Murcia, Cartegena, Mazzaron areas - all have sites, is to go St jean de Luz towards Vitoria, Burgos and Madrid, with a site at La Carbrea to the north of madrid ( or Araanda or Burgos if you don't want to drive that far) then round a bit of the Madrid ring road onto the road for Albecete hence to Murcia etc.
Best day for Madrid ring road would be sunday morning if you have not done it before.
300 miles / day is quite feasible on Spanish autopista without undue stress _ i'm well above the bus pass age so try not to over do things myself. There are relatively few sites open winter and really not much to see, so suggest you get down to the sun without undue delay.
The alternative routes from Cherbourg e.g via Clermont ferrand involve higher altitudes in France and can be very cold. Also the road system down the east coast of Spain round Barcelona can be very busy with HGVs and worse and if you plot a line from Cherboug to Valencia the best 2straight line" is as i have described.
Finally go to www.viamichelin.com and put in your route and details - you will see the alternatives and even a route chart with actual road signs if you want.
(Please excuse me if you know all/some of this - I had assumed fro the posting that you may not have done this too often before).
Do reply to this posting if you need more detail - hopefully other readers who also make a habit of getting away from a UK winter will add their ideas.