Clamping is illegal in Scotland : it is regarded legally as extortion and was outlawed a number of years ago . Furthermore , under Scots law , signage has no meaning in law unless it can be proved that it was drawn to the attention of the 'transgressor' BEFORE any alleged transgression takes place .
Even down south , I'd take the view that if a clamp was fitted to my car it , legally , becomes part of my car ( just like a towbar , foglamp or any other 'accessory' bolted onto it ) and therefore my property . I can therefore remove the car on a low loader complete with 'my' wheelclamp , or remove it by any means I see fit ( I don't normally carry an angle grinder , but usually have things like hacksaw , socket set and a good selection of other tools ) . Just don't leave any 'bits' behind as that would be littering !
Oh , and some of the signage in those car parks prohibits working on cars , so the clampers would be breaking their own rules by 'working' or 'fitting accessories' to cars in the car park .