Now here is the really difficult one. If you are on public land how do you know for sure the shelter you don't like had permission or not, and even more so if you are on private land because the shelter might be more legit than you are. It's a bit of a moral maze really and I don't the answer.
Good question
My first port of call would be the park ranger, parky or whoever maintained the site to see if they can be bothered to do anything about it.
If not and if the shelter was there used for a few weeks then i'd just knock it down, permission or not.
Might seem a bit excessive but i'm sick and tired of these arrogant selfish folks treating our outdoors like they own the place and bugger the consequences.
Fire scars, tarps used then dumped, burnt beer cans and bottles, used condoms, all things i've seen around these shelters and they scar the land for years after.
If i see a crisp packet when i'm i'll pick it up take it home and throw it in the rubbish.
If my dog does a poop on the mountain, i'll bag it seal it, take it home and dispose of it.
If i see a builders sheet of course i'll remove it, why not?
Yesterday i retrieved what must be 30 meters of tape that mountain bikers had used to mark out a course over 4 months ago and just left it.
People just don't seem to care now days, they'll use the outdoors exactly how they want and bugger the consequences.
If we all started going out and building shelters how long do you think it would be before they'd be an outrage and massive clamp down?
One person builds one of these scars on the environment and leaves it up on a farmers land and leaves it up.
6 months later a responsible wild camper approaches the farmer to ask permission to camp in hos woods and gets a mouthful about "his types".
It's not exactly a dramatic exaggeration is it, it's happening already.
And for what, what exactly does this selfish person gain by leaving their shelter up?
Absolutely nothing.
If it's raining then chances are they've taken a tarp anyways, if it's not raining then a few branches leaned against a tree aren't going to do much in our UK climate.
Unless someone can offer me something that carries more weight than the feeble excuses i've read so far, i cannot see any gain in leaving up one of these monstrosities on public or land you don't have permission for.
We used to have hippies on our farm years and years ago.
They kept to themselves, cleaned up after themselves and bought milk from us.
After they left another lot came and absolutely destroyed the place, they cut branches off a 200+ year old oak tree that destroyed it,
Next time....... nope there was no next time.
If your wild camping in England then you have a social responsibility to us fellow bushcrafters to set a good impression.
Good impression = leave no trace
In my opinion.