In manchester I've lost count on the different type of cars they're using these days but I can defo confirm they've got a few silver bmw 5 series and these arent your normal unmarked cars either, they look totally standard! the police have been very crafy with these by hiding all forms of tell tale signs as much as possible. I was so shocked when I seen one with the lights on, if it didnt go past me with the lights on I wouldnt have been able to tell. Craftiest part is the rear strobes on the silver 5 series are built into the rear clusters so u cant tell they're even there!
A lot of the police cars over here are normally MX plated even unmarked but I've seen some that arent plated like that as well. Also surprisingly seen a partially stickered up GS300 with lights on the roof as well lol.