We've found that Sea France, Dover - Calais, has offered the best deals in the past.
Yes you can get to Santander and Bilbao from Plymouth and Portsmouth - not cheap but then you don't have all that driving. Brittany Ferries, P & O, and Accion TM (I think they are) but they ditched us early this year because they 'had to' use the ferry somewhere else - quite cheap fares though if they had actually got us back home.
A trawl through the various web-sites will give you seasons and prices I should think.
Also motor homes are allowed on Chunnel - Folkstone to Calais
Get out there and travel - you know you want to.
John
PS Know nothing at all about the northern routes - sorry