I've only your paraphrase of it above, but it seems a weak argument looking at the CCTV footage; there is a considerable delay and inaction from the guy who was "grabbed" before he starts throwing punches.
If you've just head butted someone, then you're grabbed from behind and are "terrified for your life", you don't turn around, see that it's three police officers, then start a fight because you think they're gonna knife you on the spot
His adjacent mate was well aware of the police presence ahead of them even touching him; if he didn't say "look out, it's the rozzers" I'd find it very unusual