Few years since I was in France on Bastille Day. Then the bakers and small supermarkets opened until about midday - a bit like UK on most Sunday mornings years ago.
So you should be ok for basic supplies.
Supermarket filling stations will probably have card operated pumps available but that's all. Most of the larger ones Carrfour, intermarche, etc pumps now take UK credit cards. you have to authorise the transaction before filling, but the pumps have something like a hole-in-the wall screen which offers English as a language choice so it's not much of a problem.