I read (whilst browsing the Times archive for 1913) that Section 14 of the Ancient Monuments Consolidation and Amendment Act 1913 "provides that any person who injures or defaces any monument of which the Commissioners of Works or a local authority are the owners or guardians, or which is the subject of a Preservation Order.. shall be liable to a fine not exceeding £5, in addition to the cost of repairing the damage, with the alternative of imprisonment, with or without hard labour, for a term not exceeding one month."
What applies these days? There must be something similar - or just this updated with a bigger fine. I hope the hard labour's still in there though.