The BBC calls them cowboy clampers.
Apparently it is legal for anyone (claiming property rights) to place tickets on a vehicle and demand payment for use of space. More interesting is that people in the UK actually pay tickets issued by random private entities that are not "registered" with any authority.
To prove how easy it is to do that, we purchased two "DIY parking enforcement kits" on the internet.
One cost £60, the other £30.
For that outlay, you get a couple of signs to warn motorists that the land is private, and that parking is not allowed.
One set of signs indicated that the fine would be £90.
The kits also come with a book of tickets, and waterproof envelopes.
To complete the effect one of them also contains a yellow high-visibility jacket.
If the parking enforcement firm is registered with the British Parking Association (BPA), it can track down any offenders through the DVLA computer.
If it is not registered it can just chance its arm by placing a ticket on your windscreen.
30% of offenders just pay up, with no questions asked, to avoid the hassle of challenging the ticket.
It says a lot when 30% would rather comply than deal with the "hassle" of refusing to pay. That data might be skewed, however. There was a problem of wheel clamping. That is now illegal, so only tickets will continue. Far less hassle to refuse a ticket than remove a clamp. Still, my guess is this entire scheme would go nowhere in most other cultures or even lead to revenge and counter-measures like property vandalism.