It’s tricky to police a major protest. The Met have a tough job. Lost in a huge crowd of people exercising their absolute right to peaceful protest, are a tiny number of people exercising their idiotic urges to smash stuff.
Kettling innocent people is not the answer. It is a surefire way of destroying trust between police and protesters, it gives moral authority to the troublemakers and, above all, it is a denial of the right to protest, a fundamental, essential part of a functioning democracy.