He is a highly trained police officer and an armed one at that, he has a guy lying face down on the floor with his arms behind his back, yet still volleys him in the head and then stamps on him, that's the action of someone out of control.
As said before, arrest them get them in a van and give then a going over, no one would be the wiser.
The only thing that matters is the ones who started this are charged and jailed, they are lucky they pulled this off in the UK, if it was the USA or many other places in the world they would already have met their maker.