If they can't get planning permission to put fences up in the gardens I can't see them allowing them to build a coaster over the valley. I think at the moment there is enough coasters at the park unless they plan on removing 1 first. A kids coaster going though the farm would be good. Maybe replacing the same layout as the nutty ride.
I don't see why they couldn't get planning permission for a cross valley coaster in the woods behind the gardens, the gardens are listed, the woods aren't
Also there is NO SUCH THING as enough coasters, the more the better. A large ever expanding range of coasters will never fail to draw the crowds