Northern Ireland has the most independent police complaints system in the United Kingdom, and the difference is structural: complaints about the Police Service of Northern Ireland go to an independent Ombudsman, not to the police themselves.
The Police Ombudsman for Northern Ireland
The Police Ombudsman is independent of the PSNI and of government, and receives complaints about the conduct of police officers directly. In England and Wales a complaint normally goes to the force first, and only reaches an independent body on review. In Northern Ireland it starts independent. That is the single most important thing to know if you are comparing the systems.
The Ombudsman investigates complaints, and also investigates the most serious matters including deaths following police contact, and can refer files to the Public Prosecution Service where there may be a criminal offence, and recommend disciplinary action.
What it can and cannot do
The Ombudsman investigates conduct. It does not award compensation, and a finding in your favour is not a payment. A civil claim is a separate process in the Northern Ireland courts, with its own strict deadlines that run whether or not a complaint is still being investigated.
Historical cases
The Ombudsman's remit includes historical matters, and a substantial part of its work concerns incidents from the Troubles. Those investigations run on a different timescale from a current conduct complaint, and the office publishes its reports on them.