What is the best / cheapest ferry from UK to Spain?

I am moving to Spain and thought that getting a ferry would be a good plan rather than drive through France. I have been looking online and have only found a route with Brittany Ferries from Portsmouth or Plymouth to Santander for �269. Does anyone know of other ferry companies that travel late Oct / early Nov from preferably Portsmouth, but I would be willing to go from Plymouth also, to either Santander or Bilbao? The P&O website doesnt seem to let me do anything for that route so i'm not sure whether they do it anymore.