At the coronation of William the Conqueror the soldiers were really nervous, no one was sure how people would react to William's coronation for a variety of reasons. There was a large crowd outside the church. At a certain point in the ceremony was the acclamation where the people are asked if they accept the king. This was asked in French, then English. Somehow the Norman soldiers got confused at this point and thought this was a start of a riot and began to attack the crowd and set some houses on fire. Some of the more enterprising Norman soldiers took the opportunity to loot nearby houses. The ceremony fairly abruptly ended at that point with no one inside being sure what was going on.

So I guess the MET can point to that as an example of historical precedent and how they handled themselves with relative restraint.

There is also not a really deep right to protest in British law. The right to protest comes from their signing the European convention on human rights (they have some law called Human Rights Act). It is not enshrined fundamentally as in the United States afaik.