Shit, hadn't seen that. This stuff needs to be circulated as widely as possible to combat the police story that they were simply defending themselves.
You are confusing two aspects:
(1) : the police (as an organisation) are invariably correct when they say that they (as an organisation) are not the first to act violently and that their use of force (in the form of tactics used by them as an organisation) is invariably in an attempt to prevent disorder, violence and crime and / or to protect themselves (as an organisation).
(2) : it is patently obvious from many of the pieces of footage from any number of disturbances for many years, that an
individual police officer, using force as part of the police (as an organisation) deploying a particular tactic, of which they are but one officer, is frequently
not defending themselves (as an individual) from the particular individual protestor to whom they individually apply force whilst they
are defending themselves (or attempting to prevent disorder, violence and crime) from the protestors (as a crowd).
The law is clear on the use of force at the individual level: an officer using force must be able to justify it's use as against the
person to whom it is applied.
It is less clear on the use of force, justifable at the higher level of police (organisation) v protestors (crowd), by an individual officer on an individual protestor. I am not aware of an particular case in which the precise point has been considered by the appeal courts. I have seen lots of lower level cases - magistrates / Crown Court - where the higher level justification appears to have been accepted by the magistrates / jury as making the individual use of force lawful ... but I am not at all sure that that would be the judgment of the appeal courts (where I would expect them to draw some sort of distinction between the inevitable need for the police (as an organisation) to apply force to the protest (as a crowd) as part of their fulfilling their duty to prevent crime and maintain the peace and the level of
individual force by an individual officer on an individual protestor which would be permissible simply on that basis (probably restricted to common assault level, and probably based on the principle that by participating in a demonstration you consent to the possibility of that level of force possibly being applied to you by the police (and by other protestors, pushing you around, etc.) in the same way that you consent to common assault as part of any contact sport).
The "debate" being had at the moment about "who started it" and "we only became violent because the police kettled us" is overly simplistic and will get nowhere. The complexities of the situation, and the question of how the group level justifications for the use of force translate into the individual
actual uses of force by one member of one group on one member of another need to be acknowledged, considered and, ultimately, tested properly in the higher Courts.