I've taken the Santander ferry several times. It's a great way to start your holiday, and to wind down after it. It would make a nice break given the motorway slog from Edinburgh to Plymouth. Food on board is good and cheap.
As has already been pointed out, the Picos start almost right outside Santander. I know you said you wanted to abvoid the cities and the Costas but Spanish cities are fantastic places to use as overnight stops. You could do worse than head for Salamanca on your first day. It's an architecturally stunning university city (30,000 Spanish students make for a lively night scene), and there are some cheap hostels/pensions smack bang in the centre. There are some hefty bars to chain your bike to. Or you could pay much more and get a hotel with underground parking.
Road signs in Spain will, surprise surprise, put you on and keep you on the motorways. I hate motorways. I found the best way to navigate the A and B roads was to know which village was next and follow signs to it. It's a bit fiddly, but you get to see the "real" Spain.
After Salamanca, Spain is your oyster. Caceres is beautiful (the central old walled bit anyway). Then there's Seville, Cordoba, Toledo....there's just so much to see in Spain. And it's difficult to go wrong with the roads.
Don't try to see everything in this, your first trip. Save some for another time. You will return.
Last edited by SpitfireTriple; 20 Mar 2009 at 19:56.