I can only endorse what everybody else has said. Roads in France will not be as busy as in the UK, and starting on a Sunday is a good idea.
Remember to carry your documents with you in the car, and you will need to check with the hire company that they have provided the legal essentials - spare bulbs and fuses, two warning triangles, and now a one-use breathalyser (they are very cheap and should be available at the ferry port).
Driving standards are higher in the UK, believe it or not, and HGV's in France have a habit of driving right up your exhaust, but do not let them bully you into going faster than you want. If you break the speed limit and get 'flashed' or stopped by the police, there is a hefty (90 euro) fine. If you take your UK registered car abroad, it is not usually possible for the fine to be enforced, but in Spain I was once taken to a cash point and had to pay the fine on the spot.
Take your time, don't be rushed by other people. Don't pull up too close to traffic lights, as the overhead gantry type may be too high, but there is usally a little one at the side of the road as well.
Driving a LHD hire car is probably better than trying to drive your UK car in France - the driving position puts you where you should be in relation to the road and is a constant reminder to Drive on the Right.
Like others, I constantly switch between hire cars in the UK and my own car in France but you very soon get used to it. I found at first that leaving a petrol station or car park was a danger point so I learned to be particularly alert.
Driving in France is a pleasure - in my area of the Alps I regard four cars as a traffic jam.
Of course, at the moment I am driving in New Zealand - on the left with all the signs in English!
You can download the French Highway Code, or just google 'French Traffic Signs' but they are mostly the same world wide.
Beware the 'priority to the right' rule which means that people entering even a major road from the right just drive straight out. It is now mostly found only on small country roads, but there is a special sign, like a large X, to indicate you do not have right of way at a junction.
Have a wonderful holiday, and just relax!