So, what you've been told is, basically, a load of bullocks. But spelled a bit differently
Aditnow groups go out on a weekly basis in more 'normal' times, some trips require technical skills, others are 'walk in your wellies' territory - plenty of Aditnow members in North Wales, so local too. As a member of the site, you'd be welcomed with open arms (metaphorically speaking) on those trips.
About the only thing you NEED to do is join 'properly' to get insurance - the link is under the 'Information and Resources' tab, marked up as 'Join the ANMHS'. Other insurance providers are, of course, available. A lot of groups, Cave Access Ltd (google them), for example, require insurance in place.
Another local group is UCET. A friendly approachable mob who seem to frequent mines close to pubs.
You will find people reluctant to discuss 'dodgy' sites with you - there is a level of trust that needs to be built up. There are sites where, if it became widely known that they were being accessed, the JCBs and concrete would be brought in to block any access.
By 'dodgy' sites, it can be sites that are on land where the owners don't welcome mine exploration - for a number of possible reasons. Some might think they are liable if someone gets killed or injured. Others might just have a 'get off my land' attitude. Some may have had people breaking fences, litter, etc left behind.
Other sensitive sites might include coal. If the Coal Authority become aware of ANY access into an old coal mine, they block it. Old coal mines are a very quick way to get very dead.
Madness takes its toll, please carry exact change