Bear in mind there are loads of unmarked cars out there being used by squads and the like + Ambulance service, BASICS doctors, Fire brigade, etc.
I drive an unmarked 530d traffic car ... but am plain clothes within traffic and not in the least bit interested in doing someone for speeding ... I deal with fatal collisions ... and sadly am kept quite busy .... so the person writing down my registration whilst I was grabbing a coffee in a petrol station was wasting their time