The officer who pushed the paper seller, who then died of a heart attack (natural causes) was a case in point. He was acquitted and probably would not have been prosecuted if he had been anybody but a police officer.
I'll assume you're talking about Ian Tomlinson who an inquest ruled was unlawfully killed by PC Simon Harwood. The coward who attacked a man walking away from police lines with his back to PC Harwood. The man who disgraced coroner Dr Freddy Patel said died of Natural Causes but 2 subsequent post mortems stated he died from internal bleeding caused by a blow.
Although found not guilty of manslaughter by a jury his own employers considered him guilty enough to sack him and offer an unreserved apology and a substantial out of court settlement to the family of Ian Tomlinson.
I've attached a link so you can see the video of the cowardly attack and the fact that no police officer offered any help to him when he was on the floor.

http://www.theguardian.com/uk/2009/a...20-death-video

IMO PC Harwood was a lucky man.