A lot of people use the Portsmouth or Plymouth to Santander route, Others use St. Malo.
As regards coming through France be aware that there all tolls most of the distance from Calais jusqu'a Bordeaux as well as in Northern Spain.
I have twice driven from Lagos to the Nantes/Brittany region in the past couple of years.
My preferred route has been Faro to Seville, Seville to Salamance via Merida, Salamanca to Valladolid, Burgos and then Irun. Irun to Bordeaux and then Niort.
From there you can strike out to St. Malo, via Nantes, or via Tours, Le Mans, Rouen, Abbeville, Boulogne, to Calais.
My reason for preferring to run down via Spain on this route is that the price of fuel is cheaper in Spain than in Portugal, and there are no tolls once you are clear of Burgos.
If you are on a budget, there are very good deals to be had at Formule1/ETAP/IBIS hotels which dot the route.They are all part of the ACCOR Hotel Group. I have stayed at the ETAP Bordeaux South, IBIS Irun, Etap Valladolid, IBIS Salamanca and IBIS Seville.
I would suggest that in Summer you do not try to push it unless you have aircon in the car. Spain will be hot.
I would run:
Calais to Irun on Day one. About 10,5 hours. (+ add time crossing from the UK)
Irun to Seville or Faro on Day two: About 12 hours.
For the route from Santander if you come by ferry, see this article on my blog:
http://bit.ly/a4HOL1
