You're right Jan, it would not be legal for any car to be on the road whilst bearing the word "police", that would have to be covered up. Likewise, any emergency equipment must be beyond use (blue light covered and two-tones disabled).
Technically speaking, the Road Traffic Act says that it's an offence "whether working or not" but I'll find a way round it.
A friend of mine knew of my passion for classic cars and managed to come up with the roofbox and blue light. They aren't wired up (but can be quite easilly), just placed on the roof. I also have two tones for under the bonnet and another friend in the signwriting business (for the old "jam sandwich")...my only torment is whether to do it or not!
Having looked into it, not many forces had Marinas - I've heard of those Hampshire TCs (apparently, they soon became Chief Inspectors' motors [for major incidents] as the pursuit drivers didn't rate them) and even have a book depicting one of them in action! 8)