Have you used ferrysavers.com? £400 sounds extreme for the ferry booking......Ohhhhh the penny has just dropped....are you going Bilbao or Santander to Portsmouth?? If that is the case then as it is a two or three day booking it is compulsory to book a cabin....that'll be why then...
When I drive to Spain I go Dover > Calais (cheapest I have managed to get it with DFDS out of season has been £52 return using that ferrysavers website - they constantly spam the heck out of me) and in prime season June time I think it was I paid about £130 return.
I suppose you have already but have you weighed up car and trailer vs. hassle driving back?
I looked at taking my truck across and was quoted something chronic like £350 return from Dover to Calais otherwise I'd be to'ing and fro'ing every week had it been more inline with the cost to get the car across!
I have got a club Voyage membership with Brittany ferries so get a discount. The £400 includes the flight out and a gold class cabin (it's a long trip so I am having a comfy room if nothing else). Being in the south west dover is a right pain in the arse to get to so Portsmouth or Poole are good for me. When I drove the Seat back the costs were around £900 including hotels fuel etc so using the ferry from Bilbao makes sense. I am hoping I can buy it get it back and on the road for under 2 grand which if it is any good should almost turn a profit when I get bored.