You'll find the cheapest port is probably Dover/Dunkerque with Norfolk Lines (at the moment), or if you have Tesco vouchers you can use them to buy 'Deal tokens' which you can offset against the price of a crossing with Eurotunnel. However, other ferry operators are just hitting back with some bargain offers. If you're a member of the Caravan Club, or the C&CClub then check them out for ferry prices - often they offer good deals.
In April you'll find most campsites have available space - have a look at the Camping Cheque website, www.campingcheque.co.uk, or Touring Cheques, www.touringcheque.co.uk, or www.campingcard.com (ACSI). These all offer good discounts against the normal campsite tariffs, (current rates for one night are around