Ever listen to Malicorne? IIRC you're a pretty big UK folk fan so they'd probably be right up your alley. Been awhile since I've listened to them but I remember them being an appealing mix of traditional folk with proggy experimentation and some otherworldly harmonies. 2nd album is my favorite but the first 4 are all pretty solid.
Cortex's "Troupeau Bleu" has become a cult classic of sorta, partly because it was famously sampled by MF Doom. Pretty cool jazz funk sound with lovely female vox.
Will also echo recs for both Harmonium and Stereolab, although the former's best stuff is instrumental and the latter often used English lyrics. There's also Magma but didn't they basically invent their own language? What a bunch of dorks. Their music is also the type I appreciate rather than enjoy and I sorta doubt you'd be into it
I actually just listened to Laurence Vanay's "Galaxies" the other day. Lush symphonic prog from 74 but again, a lot is instrumental.
If I were you, I'd crack open poetry books by the French symbolists. Baudelaire, Rimbaud and Valery are my favorites but it's all good stuff. Among the best poetry I'm familiar with and I'd love to read it in the original French.