I'll reword it
get a couple of fake aerials on the roof, some plastic light surrounds in the rear window that look as if lights would be fitted in them (or those emergency amber strobe lights you can get on ebay, but obviously don't use them on the road) and wear a hi vis vest and people might think your an unmarked police car.
Plus from reading that link, it suggests that unmarked cars have two people in most of the time. In that case if you are on your own, get a blow up doll with a hi vis vest in the passenger seat
James, I am aware that blue and red flashing lights are illegal for none emergency services to use, but I didn't want to write too much and lose the plot
Saying that, are they allowed to be fitted providing they are only switched on on private land or shows, in the same way things like blue neons, and red lights showing at the front of the car are only allowed to be used in the same way. Or is it illegal full stop to have them, for example when emergency service cars are sold on and their lights have to be removed or painted over.
I'm not 100% but I believe the police will not bother you (or it's not illegal) if you have blue or red lights if you can only activate them from an 'inaccessible' place... Say the switch is in the engine bay or something like that. If you have the switch next to the indicator stalk or something like that (so in theory it can be used whilst driving) that can get you in deep poo-poo...
Once I had a high vis vest hanging on the back of my seat as I was driving (my estate), it was amazing how no one overtook me down the motorway for miles and miles