Having done a little digging at lunchtime, according to the Aviation and Maritime Security Act 1990 section 21C.
(1) A person shall not—
(a) go, with or without a vehicle, onto any part of a restricted zone of— (i) an aerodrome, ...............except with the permission of the manager of the aerodrome..............or a person acting on behalf of that manager or authority, and in accordance with any conditions subject to which that permission is for the time being granted, or
(b) remain on any part of such a restricted zone after being requested to leave by the manager of the aerodrome............ or a person acting on behalf of that manager or authority.
(2) Subsection (1)(a) above does not apply unless it is proved that, at the material time, notices stating that the area concerned was a restricted zone were posted so as to be readily seen and read by persons entering the restricted zone.
(3) A person who contravenes subsection (1) above without lawful authority or reasonable excuse shall be guilty of an offence and liable on summary conviction to a fine not exceeding level 5 on the standard scale."
Assuming that the runway and perimeter is "restricted", and providing that there were notices to this effect along the fence line, that would appear to indicate that any person in violation can be arrested.
