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.
Public highways
Clamping of a vehicle on a public highway can only be done by public bodies, namely; a local authority, the police or the DVLA. Normally vehicles are only clamped on a public highway if the vehicle has no excise licence, parked in no waiting or other restricted area (eg permit holders only).
We would suggest owners of private land seek legal advice either from a solicitor or the Citizens Advice via the link below with regard to the provisions they can take to prevent people parking on their land:
http://www.adviceguide.org.uk/england.htm