This bodycam one is a bit longer, which I believe helps a bit:
https://www.reddit.com/r/AskBrits/comments/1t0p591/bodycam_footage_released_of_the_golders_green/
You can see that he first refuses to go to the ground. They then taze him and he falls on his back, but then turns over to seemingly protect the knife. You can tell in your video as well as in this one, that it least one of the policemen tries to turn him over first. So he is not trying to cooperate with the police at all, let alone freely drop the weapon and is able to move at least somewhat(he clearly turned himself over after all).
They probably should have called a third policemen at that point to turn him over, as having a third person clearly worked. They seem to also have stopped kicking his head, once they got the knife and him properly handcuffed.
Still given the situation, that was somewhat decent. In many countries, he would have been killed by the police in that situation.