Evening news, re drunken brawl: One punch can kill, don't do it, manslaughter etc, etc. Panorama, use of punch to head against an unsuspecting G20 protestor looking the other way: Sometimes police have to use reasonable force if they feel they are in danger/threatened etc. Go figure.
You're also allowed to use reasonable force to defend yourself and to prevent crime. The main laws that give the police their powers to use reasonable force apply to all citizens.