To my mind, they represent a good buy but ONLY if the price was very right. Yes, they are maintained regularly with OE parts but the mechanics/techs in most police workshops are paid far less than dealer rates, so corners, as can be expected, are cut where possible.
Cars can be driven at balls out revs when cold though most Advanced Police Drivers will attempt to avoid overly thrashing them until operating temp is reached, but sometimes this is unavoidable.
Cars tend to be 'poverty spec' with basic GLS interiors despite having 'Elite' or 'MV6' on the V5, no alloys, no aircon, no sunroof etc. You do get things like ABS, traction control, limited slip diff as well as extra holes on and in the dash, heavy wear on the seats where the hancuffs and other bits on the tac belts wear through, missing factory radios etc.
Tyres tend to be Goodyear as they have the contract to supply all UK forces. Cars also usually have heavy duty alternators, bigger batteries, and extra wiring that has been simply cropped when blue lights and switches have been removed prior to sale.
Can be good value for money but, as always, your eye is certainly your merchant....
HtH
B