My question is do Morrisons have the manpower to police it? I think not somehow. How will they be able to differentiate between a shoppers car and a Devils supporters car without any tell tale stickers/clues?
Also I would question the legality and authority of anyone challenging people parking their cars and then the Morrisons bods/representatives trying to clamp them.
Regarding legality, you're probably correct, extract from
https://www.askthe.police.uk/content/Q528.htm
Private land
It is a criminal offence to clamp/block/tow away a vehicle on private land without lawful authority. Lawful authority to immobilise or move a vehicle is restricted to a number of organisation such as the police, DVLA and local authorities.
Privately owned land includes car parks, such as those at retail parks, whether or not there a fee is payable in order to park there (not local authority run car parks).
To commit this offence a person must intend to prevent the owner/driver from moving their vehicle. Therefore, clamping your own car to prevent theft would not be an offence. No offence would be committed where a driver was prevented from leaving a car park because the vehicle's exit was blocked by a fixed barrier.
BUT 2 things to bear in mind
1 - If your car is clamped when you get out at around 9:30 tonight, it's no comfort to know they shouldn't have done it, particularly if you have your kids with you
2 - Morrisons specifically asked us not to park there, as it's their busiest time of the year, there's presumably a "gentlemen's agreement" with the Devils organisation, which they could easily say is no longer valid if we don't keep our side of the bargain