Pommie
NO NO NO don't use P&O they are crap, the food is crap the cabins are crap the staff are crap the crossing is twice as long as the Brittany ferries route.
Go with Brittany ferriies from Plymouth to Santander, 18 hours, decent french cooking at reasonable prices, nice cabins and did I mention it does 30 knots.
The Pont Aven is the one.