What's the point of the cameras then?
They should be on the whole time the police are on duty, even those in the station. This will remove any doubt that they 'might' be recording while out and about. If neither the police officer or the person they are interacting with raises a complaint then the video gets overwritten or deleted.
If there is a complaint, there would be no doubt on either side that video WILL be available so the complainant had better make sure they were behaving properly before whatever it was occurred.
Other people have noted before that complaints against the police go down when the cameras are used, so why not use them?