Yes, Cherbourg Peninsula is great. I spent 3 weeks going down the west coast in August/September and only got halfway down it. The north and east coasts are worth doing as well. Possible to overnight by the lighthouse at Gatteville, and Barfleur and Saint-Vaast-la-Hougue are both Plus Beaux Villages and worth a visit. Brittany also has many superb locations, both coastal and inland.
I tend to go over in March for my early trip, sometimes late March, sometimes mid-March, and the weather varies from year to year, sometimes far kinder than the UK but on the odd occasion requiring a fast run down to the Med. On the western side of France, if the weather is wet/cold to the north of the Loire, it is often better south of the river. I have often found t-shirt and shorts weather in the Charente Maritime in late March, though the occasional year has required me to travel somewhat further south.
I have seldom encountered any issues in France with aires or wilding spots being full, even in August or on public holidays. Definitely make use of the aires, but bear in mind that they are generally not to be treated as campsites, so no awnings/tables/chairs out, though the "rules" are somewhat more relaxed on some of them - generally just a matter of common sense. In general, aires cannot be pre-booked - just turn up and find a space - though there are a number where this is possible. Aires operated by CampingCarPark require you to purchase a card that can then be loaded with funds to enable you to enter and pay, Many aires are entirely free, some charge for parking, some charge for services, requiring payment using coins, tokens (obtainable by different means in different places) or bank card.
The WildCamping Location App has a vast number of locations in France, some aires, some wilding, and many of them in superb spots alongside lakes and inland waterways as well as on the coast.
https://www.campingcar-infos.com/view.php is also useful (and there is also an Android app that costs €5 for 3 months if you want to use all features). On the website, just click on the magnifying glass in the top left corner of the map in order to search by name, coordinates, etc,
As for what to take: passports; UK sticker; hi-viz jackets; warning triangle; vehicle papers; travel
insurance. Spare bulbs and first aid kit can be a good idea as well. RHD should not be a major issue unless you plan on rally driving. High driving position gives far better visibility than if you were in a car, so only HGVs might be an issue, and I've seldom found them driving much slower than I would wish to in any case.
