You don't tend to get deals on these longer crossings. There are some offers with Brittany Ferries for the Western Channel, though the Camping Cheques website,
www.campingcheque.co.uk, but nothing for the crossings to Spain I'm afraid.
The Caravan Club might be your best bet, if you're a member. Sometimes if you book sites through them then you can get a discounted ferry crossing, but not the huge savings you'd like!