It's reasonable for an employer to decide what apps are allowed on a work phone, especially when it is a police work phone,and when the app's main purpose is tracking.

Now, in terms of protection, testing and self-isolation, the police are on the front line, like NHS staff are, and I think that they already have strict procedures that are strictly followed.

Therefore, all in all it sounds like that app on a police work phone is lots of risk, little benefits. They can install it on their personal phone if they wish.