Why not write to your local chief constable and report the driver of the unmarked car. You could say something along the lines of "deliberately slowed down to below the speed limit just to annoy me", "barely left me enough room to pass on the inside", "made threats about me losing my licence". You could then ask for confirmation that the copper concerned would be moved from Traffic duties, to which he is so clearly unsuited, into a job in which he would be employed catching proper criminals. That should make your point clearly.