It is nice to see the BBC leading their news with extended video footage of the Manchester Airport incident which adds some of the missing context. Andy Burnham did caution people not to make judgements as there was more to come. This detailed breakdown of the video by a professional explains why the kicks to the head occurred. In short the Police officer who delivered the kicks probably had not processed that the man on the ground had been tasered because two seconds earlier the man had been on top of him and the man's hand had been very close to his exposed holster. The Police officer's focus was the guy's right hand.

I think you have to click to watch it in youtube but it is worth it for the breakdown.