Well, yes.Like with most threads on here, stamping on someone’s head is neither a guarantee of going to prison nor are the police free to stomp on whomever they want without consequence.
It depends on the context. That’s been looked at now and has been decided that the officer did not act disproportionately in the instance.
Whether you agree or not with that decision is subjective and ultimately moot.
For a moot decision, some people seem very upset at anyone who doesn't agree with them (to the extent of making up stuff that they can be abusive about).
So when can the police stamp on someone's head and it be disproportionate? After any assault on them, or only when they could have shot them instead? That seemed to be one argument. But it's a poor do if in a thread on "Policing in the UK" we can't calmly discuss police powers and what is "self-defence", proportionate force, or lack of control.