There are significant issues over police accountability. The deaths of Ian Tomlinson, and DeMenzies happened because officers were not afraid of being held accountable for their actions.
As enforcers of the law, they should be held to a higher standard than the general public, not a lower one. Good intentions and 'beliefs' are not usable excuses under the law for anyone else, so they shouldn't be for law enforcement too.
When we hand police officers warrant cards, we expect the laws to be upheld, and we as a public place our trust in them to know and enforce the laws.
A new modifier 'under colour of law' should be pushed for, to deal with officers that (ab)use their position to commit offenses. It would count as an aggrevating factor in sentencing.
While UK police are generally better than their counterparts in other countries (especially the US) when an officer does go over the line, he needs to be dealt with as any member of the public would be, not handled with kid gloves because '*he's one of us*'.
Any investigation into an officer by way of a complaint should be treated like any other criminal complaint. Current complaints procedures stop if an officer resigns, while members of the public can't '*resign*' from criminal complaints put against them.
Finally, any deliberate action by an officer that results in a death should be treated as a crime, unless it can be proven to be an accident (eg car crash) determined by an independent body. Any deliberate action designed to kill someone that turns out to be innocent (eg DeMenzies) should be treated as a crime at all times, with no exception/qualified immunity)