The police often drive below the speed limit for many reasons, including that they see more that way. They are often surprised that nobody wants to overtake them. Of course if they are driving at what they consider to be the maximum safe speed (not the same as the speed limit) they might be interested.
It's just my dad always warned me never to overtake one, but never explained why. He also told me he'd never buy a red car, as apparently they attract the attention of the police. He was driving a Rover 200.
I reckon slow moving police cars cause as many jams as idiots using the break pedal on the motorways! I agree with everything everyone else said. I drive a red car and I've never been stopped - but then the police have never seen me trying to park it! :-p