To be honest it's not the antennas that make the Police vehicles stand out. Only the traffic cars that have the DF kit (4 antennas needed at least.) And that is only for those cars which are equipped to find stolen cars with trackers fitted & not all traffic cars can do that.
The current Tetra radios do not need an external antenna, so most Police vehicles have few if any antennas fitted.
It's things like dashboard clutter due to the various bits & pieces the Police fit, such as ANPR & the fact that the boot is full of cones & all the other bits, that make their vehicles sit lower at the back than most other cars on the road, that make Police cars stand out.
Often even the cameras fitted are discreet & far from obvious.
I would simply put, say 3 micro mags on the roof & leave it at that.
As a magician, I would say. DO NOT, try to copy what you see on a Police vehicle. Try to suggest it & leave it to the viewer to fill in the gaps. I could could go into detail & tell you the psychology behind this, but basically. You want to exploit the human minds pattern recognition weaknesses. The mind will see part of something & fill in the gaps, due to previous experience. And allow the mind to fool it's self, as nothing is stronger, than what someone else's mind tells them is true. So do not do the work yourself, let them do it for you.