Brit cops are not stupid. They are thinking what the LAPD thought during the Rodney King riots: "If we get out there, we will just be targets. We will not be allowed to open fire on rioters, and if we do it in self-defense we will go to jail. Time to check out that new doughnut shop on the other side of town. Call us when you're ready to give us realistic Rules of Engagement"
During the LA riots the police were busy protecting the firemen who were trying to prevent the entire city from burning to rubble.