I'm jealous. I'd love to be taking my VW to the Continent but it hasn't been built yet! Def doing it next year, all being well.
I've taken previous cars to France and further afield for the last fifteen years, on and off. A thoroughly enjoyable experience every time. You'll get used to driving on the right in no time, and strange as it may sound, when you come to your first roundabout it just feels natural.
French drivers (at least I've found) are actually very forgiving of UK plates. You'll find assholes in every country, but most of them will recognise you're on unfamiliar roads and will show you courtesy and give you a wide berth for fear you prang them.
Gendarmerie are quite keen; I've seen many more cops on the roads as each year passes. But on the autoroutes you will sail along at 130 "ish"

kph effortlessly in your motor without incident. It'll be a great long run on practically deserted stretches at a time. Just remember to slacken off somewhat if it's raining as the speed limit drops to 110, and watch for the speed cameras on the central reservations.
If you are planning on an overnight on your way to Marseille, you could do worse than stopping round about Dijon. Pick a Burgundian wine village and relax...
Hope you have a fab time and look forward to hearing all about it.