Save 20% on bookings between 26/4 and 11/07 for parks in Brittany only - although there is one in Normandy under this deal too.
Yelloh are a large chain of caravan/camping villages based in France and Spain, with great settings and facilities for all the family to enjoy. We have booked 2 weeks including half term at one of their parks in Brittany for £400 (in a 4 to 6 berth caravan). I just did a quick check and some of the parks have half term availability for just under £200 (not including travel).
I know it is not as cheap as the Sun holidays but at least this way you can get an extended stay, and also guaranteed half-term if needed.
The cheapest ferry option we found was Portsmouth-Cherbourg which goes on Sundays only, costing £213 for car and 3 passengers. Book before 5pm tomorrow using code SAVE10%MAY for a further 10% off ferry prices. Not sure if it works though as we didn't think to check on here for codes before booking...d'oh!!
If you are lucky enough to live on the South coast and don't mind a bit of a drive this could be a really cheap option to get away for half term.