Check out the Limousin. I’ve got family there and have done some fantastic MTB rides in the area. The Monts de Gueret have hundreds of km of marked track, much of it very wooded so similar to the North Downs with a few more rocks and hardly any people! there are the usual red, blue routes etc so there should be something for everyone including a couple of really fun DH tracks.
here’s a blog entry my sister has written.
Stratobiker off here lives near the Monts de Blond which also offer some fantastic XC rides.
The journey to Mimizan or wherever you’re heading isn’t bad either