I’ve got no idea how much the Vegan Society (or other orgs) charges for certification. As you said, plenty of other labels/logos they can use.

I do occasionally see reports of companies labeling a product as vegan when it isn’t. Usually from someone not understanding what vegan means (like honey). But those are pretty rare in the grand scheme of things.

Unfortunately I’m not sure how good a case someone would have for misleading claims without any legal definition on what “Vegan” means.