You'll find very few savings on Hull/Zeebrugge, especially if you're trying to book at this time of year. The Caravan Club can save money, but it won't be a lot. You could try Ferrysavers, or google 'cheap ferries' and see what comes up, but personally I would be very doubtful that you'll save a great deal, especially if you're taking a trailer or caravan.
If you want a cheaper Hull/Zeebrugge crossing then you have to keep your eye out for the early offers, sometimes 'caravans go free' or similar, about October time - otherwise there's no price war on this route, so no bargains to be had I'm afraid.
