I have driven down several times in Deember. The only time I had problems was in 2010 when it was bad in England and Northern France. Unfortunately I had an accident in Rouen and had to spend Christmas there but when I continued my journey after Christmas the roads south were no problem.
I generally don't use toll roads and from Calais go via Rouen, Tours, Poitiers, Bordeaux, Pau and the free tunnel under the Pyrenees at Canfranc and in Spain via Jaca, Huesca, Zararagoza, Teruel, Valencia, Murcia.
I have just booked my trip this weekend departing on the 5 December.
After posting this reply I have just seen you want to take the ferry to Santander. I have also done this route in December.. I would caution that the crossing can be quite rough at this time of year and on one occasion the crossing was cancelled twice as I was about to leave home for Portsmouth and it was finally cancelled completely and they offered me Portsmouth to Cherbourg which I eventully took as I could not get another competitive channel crossing at short notice. But I think that was the roughest ferry crossing I have ever had and couldn't stand up until the ship had docked.
This message was last edited by KJL on 02/11/2015.