We don't have a shoot to kill policy,it's a false statement bandied about by those who don't understand that firing a high powered weapon at the central mass of a persons body is more than likely to kill.The same people ask why not shoot in arm,leg,shoot to wound etc.It is virtually impossible to do this under the stressful circumstances of discharging a weapon at another human being,they've been watching too many films.
The ACPO policy on Firearms is best described as Shoot to stop or neutralise the threat,hence central mass.