Your humanist marriage is legal everywhere in the world, but if one or both of you are French citizens, you’ll still need to contact the French authorities before your wedding.
I’m looking forward to conducting the wedding of Anne-Laure and Henry later this summer, and it was Anne-Laure who told me about this. No matter where your wedding takes place, in France or outside its borders, the French state needs to record it. If you’re marrying in the UK, then you should contact the office of the Consulat général de France à Londres five weeks before the wedding so they can post the Banns.
The process is explained in full (en français, naturellement) and you can access the relevant documentation here
It’s probably also a good idea to check out this site if you’re thinking of having a baby any time soon, as she or he will need a passport to get into France too.
Merci, Anne-Laure, et bonne chance tout le monde!