Have a look at the UKCS website - there's an article under Caravanning and Camping Abroad called 'Advice for continental first-timers'. Tells you everything you need to know, but obviously is not specific to Brittany.
I think it would be worth your while joining the Caravan Club - the saving on ferries alone is more than worth it to us! Get quotes from them, as well as from the ferry companies themselves, for the same ferry crossings, and see what you can save. They can book sites for you, too, which is helpful for first-timers.
Dover/Calais crossings are cheaper by far - but then you've a long drive on the other side. However, French sites are used to overnight stops, so if you can make the journey part of the holiday then crossing via Calais is not a problem.
You'll find the last two weeks in July and the first two in August can be very crowded in France, so if you want a popular site then book quickly. However, if you can go the last two weeks in August then you'll have no problems, and may not even need to book at all (useful if the weather is bad, as you can move on when you want!).
Driving on the 'wrong' side is easy if you remember to 'keep your bum in the gutter' - and if you like, get the children to recite this when you go around roundabouts, pull out of junctions, petrol stations, etc. Buy petrol in supermarkets, it costs less - but not when you've the van on the back as the exits can be very tight. Roads in France are less crowded than in England, but there are more toll roads. There are many more places to stop, with 'Aires' on the autoroutes about every 10 miles, and picnic spots on many major and minor roads, which have tables and benches set up for you to use, plus room for caravans to stop, and litter bins. Use mappy.com or viamichelin.com to work out routes (you can select toll-free if you opt not to pay tolls).
Get hold of all the campsite guides you can, including Eurocamp, Canvas Holidays, etc. You can also get brochures from the French Tourist Office, and see what sort of sites you fancy. France has loads more variety of sites than we do - from large all-singing, all-dancing (but still very professionally run, clean, and with good facilities) to small 'a la ferme' sites' perhaps with only two or three vans in the orchard! There are municipal sites, run by the local mairie (or town hall) to encourage visitors to towns, and these are usually central and quite inexpensive.
Have a great time - and enjoy caravanning in France!!!
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