There are lots of clever people out there happy to sort it out for a sum equivalent to the GDP of a small nation. There'll be problems - last minute extensions and I certainly don't underestimate the cost to the industry. But I think you'll find FR will still be here when we leave, and if by chance I'm wrong - someone else will fill the void. It's the way the world works.
Sorry, Compton I posted before I read yours. All I know it was I damn sight easier to move around the continent before we joined the EU.