If the cars are not on SORN and are not taxed then they can be clamped then taken away and scrapped by the DVLA. It does not matter whether the land is privately community owned or is public land. See: www.gov.uk/report-untaxed-vehicle/ If cars are not on SORN then they must be taxed and continuously insured (though they may be insured under the garage's trade policy that does not have to list them all). Non insurance is a more serious offence than non taxed, but the police will not act on that unless the car is seen being used on public roads.