Do you want a solution that relies on having 3g / 4g data access, like some of the French mapping sites require, or something you can load onto your device? If it’s Garmin, I guess you’re after a map that you can load onto eg an Edge as a file?
Try https://talkytoaster.co.uk/maps/
..the UK coverage is good, and I’ve also used maps for Switzerland and Austria. You could also look for Open Source Mountain Bike maps.
The Garmin Edge 1000 I just bought came with Garmins bike map of Europe, which is OK but lacks contours. Not sure if you can get a copy of that from their site?
There are other ways of creating maps manually and loading them on to Garmins, some of which are pretty complex. Try Talkytoaster first maybe.
Good luck!