There're two distinct things here: the sentence and the employer's response (setting aside the fact that the employer is the police).
Neither is acceptably severe, but it would only be possible to appeal the sentence if it wasn't in line with sentencing guidelines. I wouldn't be at all surprised if a first offence for an "upstanding member of society" plus a possible association with the same lodge/golf club as the magistrate is typically in line with what he got.
However, I would have thought that this would count as gross misconduct where an employee has misused training gained during their work to assault a member of the public, and bringing the reputation of his employer into disrepute. However, as his employer is the police, maybe they regard their reputation as untarnishable, at least as far as the people who matter to them are concerned.
The only upside to this is that some more of the public might begin to realise that the police are institutionally violent and sexist as well as racist, but I suspect that the bad apples myth will endure.