I often think the crims, or terries, believe that Police have to wait to be shot at.
Fortunately this is an exert from the UK rules:
To prevent an immediate threat to life by shooting to stop the subject from carrying out their intended or threatened course of action. In most circumstances this is achieved by aiming to strike the central body mass (i.e. the torso).
So as long as the policeman, at that time, has a reasonable belief he or others are about to be shot at, they can shoot. This was backed up recently by a case, where crims were trying to get another crim out of a prison transport van. Police knew, and shot one dead. Only a replica gun was found in the car, back seat. Policeman cleared, he had been briefed that they did have guns, and would use them.