He did slightly more than singing that at police, according to the BBC article you link to:
These sentences are still insanely disproportionate ththough.
Yeah, I wish people would stop posting clickbait without context, it doesn't help. Even so I think even just shouting at the police would be enough for the sentance, or atleast not far off it.
The reality is that all (from what I've seen) of these convictions do come under Violent Dissorder. The problem though is that Violent Disorder is a pretty bullshit charge and the clear two-tiered application of the law is showing this.
Slighy PL: I've been charged with it before and was given a suspended sentance, something I was told I was very lucky to recieve - for pushing somebody over...
A mass brawl happened outside of a pub and everyone involved was given the same sentance, regardless of whether you were pushing and shouting, or stamping on heads.
Hearing a completely out of touch judge theorise about the brawl was quite eye opening, and reading about the 'gang warefare' in the local papers was a laugh. In reality it was a few people pissed up getting into a fight, and friends and family becoming involved trying to break it up, leading to a large scrap.
The point being that someone within a crowd of lairy people, screaming at police, and especially being caught on camera pushing someone, can 'rightfully' be jailed for ~2 years. The judges are likely completely divorced from reality and believe they're jailing thousands of thugs.