It's clear from conversations with fellow expats, chat on various expat forums and Facebook groups that many currently travel by car/ferry on the Portsmouth/Plymouth to Santander/Bilbao route. Upon arrival the Spanish authorities seem to be a law unto themselves, not interested in any kind of customer service. Given the choice people would prefer the ferry just to continue on to Portugal itself and cut out having to drive through Spain. 

Please Brittany Ferries, listen to the needs of your customers, provide a direct service. No preference as to where, anywhere on the south coast of the UK to anywhere in Portugal. Remove the requirement to transit Spain. Conduct a survey of your current passengers, find out their final destination.