P&O give 30% off if you quote MCN, their number for Portsmouth-Bilbao sailings is 08702-424111.
Brittany sail from Plymouth to Santander, no discounts that I'm aware of? Their number is 08705-360360.
The Bilbao ferry takes 26 to 30 hours, the Santander ferry takes 18 hours as Brittany have recently introduced a new fast ship.
Personally, I find Brittany to be a better experience than P&O. Phone both, there's only about two sailings per week from each port.