Interesting hypothetical as I was thinking of my trip over to a nearby mining structure when reading the first post in the thread.
When I was posting the picture it did cross my mind that it might look like I was quite close, but it’s the perspective of such a big structure… I was nearly 60m away.
Could I get closer? I would say no.
As @kvetner says, it all comes down to “control”, and I would be hard pressed to argue I had any actual control over the structure. Though I had clear line of site to the base and nobody would be able to climb up without me knowing, if a maintenance team arrived I’d have zero authority over them or their actions. Also, I could see the bottom when I took off, but until the drone was up high enough to be my eyes, I didn’t know whether there was anybody up on the platforms already. So 50m it was. Had I contacted the trust in advance and got their permission to control the site that would be a different kettle of fish.
As an aside, my lack of control of the site meant I had to cut the flight short. When I started I was the only person there… but after I’d been up about 10 minutes a kid on a quad came racing straight through my landing area. There and gone in seconds, but when he came back a couple of minutes later I realised he was doing loops of a footpath. I had no authority to stop him, so I packed up and headed home.