Defence of another is a common law defence to assault. If someone went toward me with a knife, and you were present, you would be entitled to use whatever force was necessary towards them to prevent them from attacking me, including (in some circumstances) using that knife upon the attacker. I don't believe that should be a difficult concept to grasp. You are lawfully allowed to use reasonable force, when you perceive a threat to another, as long as you reasonably apprehend that force to another to be a clear and present threat. The two concepts of defence of another and reasonable force are not mutually exlusive, as you appear to suggest. Quite the opposite, they are both key components to any defence to a charge of assault in these circumstances.
Based on what I have seen, I do not believe there was any way he could have known with any certainty that she was not a threat, given he time he had to react along with the location and people present. Did he know for sure she was a Greenpeace protester?
I believe his reaction could be held to be a bit OTT, but that does not make someone guilty of assault unless a jury is sure that person did not reasonably believe there was any threat to another, especially given the limited amount of force he used. If he'd stabbed her with a bread knife, then the position would be manifestly different.
When I said 'acceptable' I meant in terms of criminal liability, not my own perception. I should have made that clearer.