There's no empire to protect, just a different way of doing the job.
In balance, in UK it's considered better for an experienced pilot to be trained in the type of flying the police require, than for a policeman to gain the required flying hours and broad experience. There is nothing to prevent the pilot, whatever his background, getting more physically involved if necessary but in general the best place for the aircraft is in the air, backing up the ground team.
As far as "empire protection" goes, the CAA sets out the minimum flying hours and experience required. This is increased by some police authorities. In the case of the unit I flew for, the minimums were doubled (to 3,000 hours). If a police officer had those hours and the required experience, I'm sure he would be very much welcomed as a pilot.
Using a professional pilot (usually ex-military) to concentrate on the professional aspects of flying, leaving two police officers to concentrate on their specialist subject, works very well. One officer is the camera operator. Being a police officer he knows exactly what is required to maintain good evidential continuity. The mission commander / comms officer is also a policeman /woman.
There was a well-publicised fatal accident some years ago in UK (an Optica aircraft, Hampshire policeman flying, IIRC) that was instrumental in changing the mindset that police officers should be doing the flying.