I can’t give any particular advice regarding the immediate vicinity, as the closest place I’ve ridden is at La Clusaz, but my observation would be that a French “Red” marked trail might be a bit of an eye opener with regard to UK “Red” trails.
I rode at Vars, Risoul, Serre Chevallier, Puy St. Vincent and Les Orres this summer. Most of the blues would have focussed the mind of a Glentress “Red” standard rider.
I’m normally blind to grading and difficulty, but this summer I was scoping trails for my 7yo, so paid particular attention. My brother and I blasted down one of the blues at Les Orres and decided that it was great at full-tilt, but there was no chance of our kids riding it. My lad has done the Red at GT, although he required towed to the top. My brother later gave it a go with his lad. Multiple faceplants ensued.
If you go easy, you should be fine, but don’t assume grading equivalence.