Gates of Vienna is generally rather eager to dance gleefully on British graves. However, in the clip shown it is clear that the small number of police present thought they were dealing with a minor disturbance involving one stroppy individual, and were not expecting the huge mob that showed up within minutes.

Compare the response here, when they were prepared.

Diffusing a situation politely - as they tried to here - is usually the best response. And we don't know what happened next - were reinforcements called to quell the mob?