What exactly were they supposed to do? They are unarmed. No riot shields, batons, guns and tear gas, and they are confronting an extremely angry mob. If the cops had attempted to arrest even one of them there would have been a full scale riot.
The vid ran for 10 minutes. It takes more time than that to deploy the riot squad. I bet London to a brick that they did get backup eventually.
But that wasn't the purpose of the vid was it? The poster had an agenda and you bought it hook, line and sinker.