What are the rules on buying & using marked ex police cars on the road?
I had a friend in Switzerland who bought a Dutch Police Range Rover but was not allowed to use it in CH. 
Basically, local Constabulary should be informed, this ensures that the car is noted on the PNC to minimise unnecessary traffic stops. Membership of a relevant club gives the car a degree of legitimacy. It can be used on road for travel to/from events/meets and for 'Battery charging' purposes. All equipment must be satisfactorily disabled when on the road with any exterior blue lights covered. Must not have the word Police visible anywhere. Technically the blue bits of the livery should be covered as retroreflective materials are deemed to be a light source wrt the Road Traffic Act Construction and Use regs. Vehicle needs to be clearly marked as 'Not In Service' whilst on the road. Should also have adequate, relevant insurance, including public liability, MoT and tax in place. Finally, a log must be kept of every journey: start/finish locations and mileage, purpose of journey, name of driver. Log must be presented on demand.
There's a set of ACPO guidelines which have to be strictly adhered to to avoid an impersonation charge, but that's the gist of it
