They may not like admitting to mistakes, but they don't like making them either. Even if you make a complaint via the media regulators it will be sent straight to the editor for comment. If you do make the complaint yourself, please remember that abusive or threatening language is likely to be counter-productive. You are going to be upset, but the chances are that this was not a deliberate act on the editor's part. A reasonable attitude gets better results.
Editors are only human.
If you have evidence to back up your complaint, tell them what it is - but remember you are under no obligation to supply personal documents or information that others have no right to see. Be clear about what you would like to see done about the matter you are complaining about . You may wish to have a correction or apology published. If so, and the editor agrees, make sure that you confirm the wording of the correction and when and where it will appear. If there is a good reason for a delay in contacting them, explain it or your complaint may not be entertained.