I find it disappointing that there are mine explorers and cave explorers, and seemingly a few sewer explorers too. When I was younger and fitter, any hole in the ground was fair game, whether natural or man-made (but never sewers!), and I would never have thought that various semi-opposing factions, some political and others just wanting their fun, would arise. I was a bit biased towards mines because the only subterranea within easy reach (10 to 20 minutes by bicycle) with no apparent access restrictions were the Blairlogie Copper Mines and the Craigend Lime Works.
On holiday of course there were invariably sea caves, a few of which I now know to be mines. Then I got a car, and Leadhills/Wanlockhead, except in the grouse season, was accessible, amongst other things, under the old regime.
What would be good would be for all interested parties to be pulling in the same direction with regard to access, which in many cases could be more liberal. But the law needs to exempt landowners from the consequences of careless explorers having accidents, and from damage due to mindless vandalism etc. It just isn't fair that some poor farmer might be sued for millions in damages if an idiot is killed underground in his land.
I don't know how well the right to roam law is working in Scotland. Last I heard a particularly obnoxious landowner deservedly lost in court and certain footpaths on his land are now available. The law does extend to rivers etc (not fish of course) so you can, for example, mostly use canoes on the rivers without hindrance. But I have no idea what the law might say about underground activities.
The only thing that would work properly in England is a well-thought-out and fair law which protected the interests of all parties. If enough people of all persuasions pulled together and lobbied their MPs we might just get that. We are surely talking of maybe 100k people with underground interests?