If the doctors in France said that it wasn't the ligament, what did they say it was?
I found these -
https://tau.amegroups.com/article/view/4458/5341
https://www.lybrate.com/topic/penis-injury-of-the-suspensory-ligament-what-men-need-to-know/b35b148ac29e2e5f7fa1c40cbab0fc3d
It seems to suggest that a certain type of ED is caused by the suspensory ligament injury, but I'm certainly no expert. It seems like people with this can become erect, but it hangs, and doesn't point up. That's my interpretation, anyway.
Do you live in London or France? If you live in London, I'd definitely get another opinion before getting surgery, and find out if surgery will be covered before agreeing to it.
Have you had any imaging done, like an ultrasound? I'd make sure that was done, too.