All the above comments are quite cogent. You've all made some very good points. Vandalism of nature hits us all in a special place, as bushcrafting and being out and interacting with nature is our passion. I feel that engaging those who have little interaction of being out and about in the wilds, in a non-confrontational way, helps open some eyes to what nature is about. Take a kid for a walk in the woods. Grab some senior drive them out to the countryside.
As for vandalism, the state has abdicated it's role as guardian of the wild. We can assume that role of guardian or steward, but that opens a legal can of worms... This is why I suggest engaging folk in a non-confrontational way...take them out to the woods instead of yelling at them or getting your blood-pressure ready to pop.
England has a particularly difficult problem of about 60 million people on an island that studies suggest should only support 20 million, the #'s here suggest that too many people in too small of a nation would only lead to degradation of the lands in immediate vicinity of large urban centers...
If you're up for it, reasonable engagement seems to be the most likely way of affecting local and immediate change.
Sorry to stick my nose in your business. It bothers me to hear what is happening there, as I used to listen to my Nan talk about the countryside near Brighton and Lewes when she was a young girl.
Hope things get a bit better.
Cheers
Alex