According to the CPS
Aerodrome Trespass
Section 39 Civil Aviation Act 1982 (Stones 7-5374) makes it an offence to trespass on land forming part of an aerodrome licensed under the Air Navigation Order. You should note that a defendant is not liable under this section unless it can be proved that at the time of the offence notices warning trespassers of their liability could be readily seen near the boundary.
I hope that they get the book thrown at them including civil proceedings to recover losses made by the airport and the airlines.
There is any number of forums to legitimately protest and create debate. This lunatic green fringe has totally failed to gain mainstream public support so they have to opt for this sort of acts.