But the difference in grading criteria means that it's quite possible to end up doing a huge, gruelling ride, with little in the way of technical singletrack in return.
Oh you mean like trail centres in UK then.
Can you guys please bear in mind the fact that France is a rather spread country with a lot of different topologies.
What will be graded blue in Nice will probably be graded Black somewhere else. And so and so.
Plus exept some local restriction, you can ride on in footpath in France provided they are located outside "Parc naturels nationaux".
The FFC-VTT are just guidelines. You should check the local councils or conseils generaux for more informations...
Plus France isn't the 3rd world you know, there is plenty of MTB forum where you could meet locals who I am sure will be more than happy to show you around.
Hope it helps.