I never said anything about self–defense. I am saying you are misunderstanding how the burden of proof and presumption of innocence works. Your original comment, referring to a police officer using force:
> If they are unjustified, then you should be able to prove that. Otherwise what you're talking about is the police having to prove their innocence.
If I allege that a police officer used force on me, they have the presumption of innocence. However, if they admit to using force, that presumption vanishes. We can assume they did it, because they just admitted to it!
The question then becomes whether or not that force was justified. In answering that question, the burden of proof shifts to them: I am presumed innocent of threatening them, and they must affirmatively prove that I did something to justify their use of force.
Edit: I never said anything about self–defense because your original comment was about the burden of proof for use of force by the police, not against them. You are moving the goalposts.