Dangerous things these raves. Subversive stuff...
From the Criminal Justice & Public Order Act, 1994 :
Sections 63, 64 & 65: Raves
A 'rave' is defined as a gathering of 100+ people, at which amplified music ('wholly or predominantly characterised by the emission of a succession of repetitive beats') is played which is likely to cause serious distress to the local community, in the open air and at night.
These sections give the police the power to order people to leave the land if they're believed to be preparing to hold a rave ( 2 or more people); waiting for a rave to start (10+); actually attending a rave (10+). Ignoring this direction, or returning to the land within the next week, are both offences, liable to 3 months' imprisonment and/or a £2,500 fine.
Section 65 lets any uniformed constable who believes a person is on their way to a rave within a 5-mile radius to stop them and direct them away from the area - failure to comply can lead to a maximum fine of £1000.