Quite was does an exclusion zone mean? Could that 200' diameter circle end up being surrounded by an industrial estate, left as a nice ornamental roundabout at its centre, but that's okay because the exclusion zone is still secure?

I would think the answer is yes. You put your finger on the problem - beyond the scheduled boundary of a monument there is actually zero protection and commercial pressure/local planning decisions can do with the land whatever they like.
You could argue that the 200 ft diameter "exclusion zone" is generous as they didn't have to provide it. (I suspect it has more to do with providing a quirky recreational space than acknowledging the monument has a "setting"). There is no statutory protection for Thornborough's landscape further than about ten feet out from it - and no sign the planning authority is willing to recognise it deserves any protection.