This is a very interesting topic - camping on someone's land with/without permission.
I own some land. Not much - just 5 acres. It's a mix of mostly meadow with a small (about an acre) area of open woodland.
What would I do if I saw someone camping on it?
Hmm...that's a difficult one. There are a lot of 'depends' and caveats tied up in that response, as you would imagine.
If it was someone like myself (whatever that may be) who was being discreet, subdued colours, tucked away, using a camping stove...well, I'd turn a blind eye, pretend I hadn't seen anything, and leave them alone. I can understand where that comes from. If they were all the above but had a fire, I'd approach them, say they can carry on, but please do not take any more wood. If they had brought their own wood then I'd let them carry on with the fire, so long as it wasn't a massive inferno and I could see they had made effort to control the fire. I'd make it clear that any trace must be completely cleaned up when they move on.
If it was a family or someone in a tent, making no attempt to hide but being careful and just wanted to camp out for the night? Then I think I would approach them and let them know they have my permission and point out all the above.
If it was someone with a massive fire, or acting like an idiot? Well, I think you know what I would do. They'd be moved on.
If someone asked me permission before actually camping there?
They'd get twenty questions, but just so I could gauge what sort of person they are (though the fact that they've made the effort to find out who owns the land and actually asked me would also weigh heavily in their favour), and so long as they abided by some rules (be discreet, no fire, no bright colours, no noise) I'd be happy for them to camp there for one night.
If they asked for regular permission to camp there, I'd say no, but let them know they could ask any time they wanted to and I'd make a decision based on several factors.
I let a farmer graze his cattle in the field, and if it was during a time when the cattle were present I'd say no, out of respect to the farmer who owns the cattle.
If I started getting too many requests because it was known that I'm likely to say 'yes' then I'd start saying 'no' no matter what.
I'd be very interested to know how other land owners here would act in similar cases.