Lets not have trial by forum. I would imagine the attack would have been over fairly quickly, we don't know where the police were and it does look like as eye witnesses have said that there was a car crash and the two perpetrators were seen to be going to the aid of a man who was injured. Not so as it turned out obviously but 'I would have done this ' etc is fine in hindsight.
You don't expect a man to be hacked to death in the street in London, you would think something else was happening regardless of what you might think you would have done.