1) Where 12 or more persons who are present together use or threaten unlawful violence for a common purpose and the conduct of them (taken together) is such as would cause a person of reasonable firmness present at the scene to fear for his personal safety, each of the persons using unlawful violence for the common purpose is guilty of riot.
(2) It is immaterial whether or not the 12 or more use or threaten unlawful violence simultaneously.
(3) The common purpose may be inferred from conduct.
(4) No person of reasonable firmness need actually be, or be likely to be, present at the scene.
(5) Riot may be committed in private as well as in public places.
(6) A person guilty of riot is liable on conviction on indictment to imprisonment for a term not exceeding ten years or a fine or both.

and another

http://www.legislationline.org/documents/action/popup/id/7345

Section 59 Public assemblies

In section 16 of the Public Order Act 1986 (c. 64) (which defines "public assembly" for the purposes of the power in section 14 of that Act to impose conditions on public assemblies), in the definition of "public assembly" for "20" substitute "2".

The 1994 quotes of trespass were entirely designed to prevent free festivals (the scientific and historical monuments bit is specifically aimed at the stonehenge festival) Further rules are in the later ones to avoid raves and other  events, but are written wider and so applicable against political protest too.

Any idiot can face a crisis - it's day to day living that wears you out.