If you come down to Cornwall, you will find all of your answers. People don't want you in their holes. If you ask them and make a fuss, they will merely look at their entrances more carefully.
Obviously, we are mostly mines down here.
As a part of my ?5m insurance cover, I made specifically sure that it covered "going down holes" to survey them for the purpose of subsidence risk assessment (under the guise of work). Even with this card pulled, people do not like the thought of holes, or anything to do with them. If it's a load of people "just poking around" or a seriously organised club with lots of special titles like a company, it doesn't change the mindset.
Stop making a fuss unless you absolutely have to. I VERY MUCH RESENT THIS CLUB MENTALITY AND "EMPIRE BUILDING". "We are all good and responsible, everyone else are a bunch of dangerous nutters, we will manage access for you and here's our BCA insurance".
If any of you waggle your willies in front of a landowner with this nonsense, or any other caving club nonsense, they will most likely tell you to F.O. They are more likely to then see that Jeeves makes sure that the entrance is sealed. This goes for the crown, duchy, private estates or Mrs Goggins with a shaft in her garden.