For me it has to be the 1980's .
I started exploring in 81 then stopped in 89. Access to almost anywhere including working mines was easy.
In Derbyshire the large spar mines of Long Rake & Ladywash had only just closed, both around 1980-81, both mines were still in the most part in good condition, with plenty of original atrefacts in situ. The still working mines of Sallet Hole & Watersaw, if you turned up to the mine on the late shift & spoke to the shift boss, he used to allow us in to watch what was going on. Most of the large drainage soughs were still open, if you wanted a wet trip! & we did.
We had several official trips courtesy of the NCB into the Nottingham coal mines, usually on a Sunday , when mostly maintenance was going on.
Up north around Nenthead, again the situation with the old lead mines was pretty good. Also a few of the British Steel spar mines were still operating, access could be arranged into some of these, although this usually invloved a bottle or two.
We never travelled very far into North wales for some reason & so did not get to the slate mines. We did however go as far as the Mold area & the Halkyn lead mines system.
The headframes were still standing on the main working shafts, with rows of small mine tubs all over the place, sadly that is not the case now, with almost everything removed.
Having restarted exploring again only last year, I have been suprised at the rapid deterioration of many of my old haunts, indeed several have little or no access at all now, many are flooded or infilled, virtually nothing is working bar one spar mine that I can think of.
The management thanks you for your co operation.