If someone is pointing what looks like a gun at me, and expresses an intention to use it to kill me in the immediate future, I think I'm justified in going for a pre-emptive strike. If circumstances mean that the only pre-emptive strike I can make is by using the gun in my hand (reality intervention: I don't have one, and wouldn't know what to do with it if I had) to render them incapable of attacking me, then such is life, and if they didn't want it to happen, they shouldn't have made the threat.
I gather that the British police force agree with me. (Not that that means they're right, or I'm right, but it's a possibly useful standard of comparison.)