Nick, the police ALSO have a right to protect themselves, and the obligation to restrain him if he's suspected of committing an offence.
If he's struggling and they feel he is likely to get away they can use force to complete the restraint.
If they are trying to restrain him, and they fear he is about to do something that puts them at risk they have the right to use force to stop it/protect themselves (even joe public can use force if they feel threatened*)
As has been said many, many times, the video only shows what the camera can see, it doesn't show anything like everything that is going on, nor does it show that the person on the ground may have just said to the police officer "pig i'm going to stick you", or that the officer may have spotted something that the guy was moving his hard towards.
A lot of people seem to forget that the police not only are allowed to use forces to restrain people, but have exactly he same right to protect themselves from harm as we do, and they are expected to put themselves in harms way just to do the job.
The police may be expected to protect the public, but that does not overrule their right to protect themselves, their colleagues or the fact that force may be required to restrain someone, no matter how bad it looks on a short bit of footage or a single pictures form hundreds.
*You may end up in court/with it being investigated, but if it's reasonable for you to have suspected that someone was about to do something to harm you, you can even hit first (and oddly enough this is pretty much what seems to be happening in this case, it's being investigated with possibly outcomes including criminal prosecution).