Hmm.
Depending on where you are going (certain large towns and cities) you may need a Crit'Air sticker to show that your car satisfies emissions requirements for driving in urban areas. Check the places you're planning to drive through.
If you have a newish car with a digital speedometer, figure out how to change it to KPH so you don't accidentally get done for speeding.
It's illegal to use any sat nav device which tells you exactly where speed cameras are in France, only the general area, but if you're using something like Google maps or Waze it should automatically adjust to conform to local laws. Speed cameras are much less visible in France than in the UK.
Make sure you have a hi vis vest and a warning triangle in your car in case of a breakdown. (And European breakdown cover, of course!)
Remember to give way to the left and bear right at roundabouts. Occasionally you may encounter a roundabout where those joining have priority; in these cases there will be line markings on the roundabout itself. (Most of them are not like this and work the same way as British roundabouts.)
Watch out for cars coming out of side roads on to your right, they sometimes have right of way.
Also watch out when you have a green light to turn left or right at a junction, because frequently there will be a pedestrian crossing on the road you're turning onto which has a green light at the same time. French urban planners don't seem to have realised that traffic lights were actually invented to avoid exactly this sort of problem. 🙃
Basically just keep your wits about you and don't assume you have right of way in situations where you would normally expect to have right of way.
More general France tips...
Always say "bonjour" and smile when you enter a shop, cafe or restaurant, and "au revoir" when you leave. Even if your total vocabulary is limited to "bonjour", "parlez-vous anglais", "merci" and "au revoir", using these words liberally will make a big difference to your experience in France. Even more so if you can manage to order your coffee/baguette/beer/wine in French. (They will be delighted with this in Spain too.)
If you want to eat in restaurants but are flexible about when, a lot of good restaurants will do a fixed price lunchtime menu that costs a fraction of what it would cost in the evening, so go for a big sit down lunch and a pauper's supper of bread, cheese and wine if you are doing one or the other.
And probably the most important tip of all, if you go to a boulangerie and order a "baguette" you will get a bog standard baguette. If you order a "baguette tradition" or a "tradition" you will pay about 20c more but get the deluxe version with nicer ingredients.
Enjoy!