Context would be relevant IF the kicking had happened while trying to subdue/contain the suspect, in which case fair enough. That's when "fuck about and find out " applies.
At the point it actually happened, lying on the floor, face down, arms still by his side, it becomes irrelevant because there was no need for a head kick. There simply wasn't. It was an action taken in anger and violence, not as part of an arrest. That's why it's an issue.