Of course they can. Although most proper traffic police are excellent drivers and excel in dealing with other drivers there are a few that seem to get ideas above their station(s).
If you witness poor driving by a liveried (or unmarked) car then you're at liberty to report it. Clearly. little action is taken in general - how would you feel if your neighbour decided to report some minor action on your part and you were immediately prosecuted?
I do think that some of the Police/Camera/Action TV shows demonstrate that Police drivers can be very poor indeed.
Why, for example try and stay on the bumper of a 1.4 Clio (when the police are driving an Impreza) just in case it takes a LH corner. Let's say the Clio doesn't and then carries on at 60mph. The Impreza driver (police) could've afforded to slow to 5 mph for the corner just in case the Clio took it. If not, you'd be back with it in three seconds anyway.
Some police drivers are a bit short in forethought. Perhaps green tinted glasses would counter the red mist