The Wrong Arm of the Law (1963)

The crooks in London know how it works: No one carries guns and no one resists the police. Then a new gang appears and goes one better: They dress as Police Officers and steal from the crooks.

This upsets the natural order of the Police/criminal relationship, and the Police and the crooks join forces to catch the impersonating Police Officers, including an armored car robbery in which the Police must help the gangs to set a trap.