Clamped or Towed on Private Land

It is a criminal offence to immobilise a vehicle with a clamp, move it or restrict its movement to stop the person entitled moving it, without lawful authority. Consent, for example by parking under signs, is not lawful authority, and contract law does not give a landowner or car park operator authority to clamp or tow. The penalty can be a fine, unlimited on indictment (Protection of Freedoms Act 2012, section 54). source

Lawful authority includes local authorities enforcing on the highway or in council car parks, and the police in specified situations. A fixed barrier you agreed to when you parked is not an offence. Someone entitled to remove the vehicle, such as a hire company, is not committing the offence. source

Keeper liability (England and Wales only): under Protection of Freedoms Act 2012 Schedule 4 the operator can only pursue the registered keeper if it follows strict notice rules. A notice to keeper must arrive within 14 days of the parking if no ticket was left on the car, or within a set window after the 28-day period following a ticket left on the car; the keeper then has 28 days to pay before liability arises. source

Private parking charge: you can appeal to the operator first, then to POPLA (BPA members) or the Independent Appeals Service (IPC members) for free; while a POPLA appeal is open the operator must not pursue you for payment. source