Hi Jonathan
Tips:
1. Are you looking at taking your own equipment - if so then book everything directly with the sites (or try Alan Rogers as Thelma has suggested - or the Caravan Club) rather than through Eurocamp Independent or similar, which can cost more. If you're not taking your own equipment and want a ready erected tent or mobile home, then find the site you want and then see what sort of accommodation they offer, directly. Most sites offer a variety of mobile homes, or tents, or cabins, these days.
2. Use a short ferry crossing rather than the longer western crossings. Our crossing *(booked at the time of the NEC Caravan Show using a special offer code) was £58.00 return, with Norfolk Lines, Dover to Dunkerque. They make these offers available at the Caravan Show and the promotional codes are often circulated on forums such as this one. It might be worth waiting until the next show in February to see if anything comes up (but this is by no means guaranteed). Play around with the timings of your crossing to bring costs down. It makes for a longer drive to some sites, though - but driving is more pleasant in France than in this country.
3. Have a look at the Camping Cheques or ACSI card websites -
www.campingcheque.co.uk, or
www.campingcard.com to see what sites are available. The problem is that in the Vendee (a very popular area with some Brits) the Camping Cheque/ACSI acceptance dates don't really apply until that bit later than in some other areas. There are sites which accept Camping Cheques, or an ACSI card, from the 15th August - but not in the Vendee.
4. Go the last two weeks in August, rather than in July or the first two weeks in August - site prices come down on some sites even if you can't use Camping Cheques or an ACSI card. Large family sites do tend to be more expensive, especially right on the coast, but if you go a little inland you may be able to save some money. If you're prepared to stay on smaller sites then there are lots of them - check out
www.campingfrance.com for details of all 11,00 campsites in France (has links to campsite websites).