I don't think there are many people here with French food retailing experience, especially in ice cream (eating being another matter). However, I know that there are mobile pizza vans and a few travelling chippies in France, so ice cream isn't necessarily a problem. That said, there could be all sorts of hygiene rules but if you have UK experience these would probably not be a problem.
I'd suggest that you should start by asking at the mairie in the area you're looking at targeting. They should be able to point you at the appropriate registration authority.
By the way, it helps if you say roughly where you are (now or planning to be) plus how much experience of your chosen career and of France you have. Otherwise people start guessing and the thread can wander off into hypothetical discussions.
Finally, your market will probably depend on whether you're looking at industrial (Walls / Mr Whippy) or artisanal (made yourself) products. See the current thread about ice cream.
Albert the InfoGipsy
"So welcome to the Citadel where the question is 'Am I?'"