We're expecting supposedly highly trained professionals who have been given the right by the state to use force against the public to act within their training and be marginally better than a chucking out time brawler. AFAIK there is no force or training in the UK where kicking someone in the head, or stomping on their head is "acceptable" or "standard".
This wasn't an extra punch when someone was down, or a single kick to the chest, it was sustained, aimed and deliberate, that is the sort of thing that whenever there is any question of "as the force reasonable" it tends to go against the person.
The weird thing is, I'm arguing that the level of force in this case seems to have been excessive, when I'm fairly sure in the past I've argued with some of the same people that an armed officer firing at someone was probably justified.