I bought a couple of ex traffic police cars years ago, Cavalier turbo's. High mileage as you would expect but mechanically sound. Don't expect built in sound systems or any extras though as everything is ripped out to accommodate the police spec leaving holes everywhere.
Expect to be pulled over by the police a lot though as they recognise the registration plates and for some reason believe the car is wanted or been involved in crime. It's only when you explain to them it was an ex GMP car they click on and realise why they knew the registration plate.
I bought mine from a bloke in Hyde all he dealt with was ex police motors I don't know if he is still about.