Thats a real German police car, green and white are the colours of the German highway patrol.
No, it's definately a fake, partly since it's rhd (if it was genuine you'd leave it as lhd - since conversion to rhd on a 1992 Passat worth about NZ $1000 tops would be ridiculously expensive and also totally unecessary since unlike in NZ, lhd cars are road legal in the UK regardless of their age. Also, being a bit of an anorak here, it looks to me to be a different shade of green!. Do like the re-arrangement of the "Passat" badge to read "Past it" though!
And the probable reason why they don't have a blue light on it (as a genuine car would) is because in the UK it is illegal for any vehicle which is not a genuine emergency services vehicle (fire/police/ambulance) to display blue lights....so they'd get pulled every time they drove it on the road. Penalty = fine (at least), possible criminal conviction, possible confiscation & crushing of car.
Some exceptions are allowed (with the necessary permits) e.g. for fake police cars to be used for tv filming or restored classic police cars to be displayed at shows etc but even these must have any blue lights covered/removed when on public roads.