So far as I have seen on this thread, what ED209 has said is absolutely correct. The use of lethal force by whatever means necessary is lawful and justifiable should the circumstances permit, and it is the juries responsibility to determine this. To state a firearms Officer should aim for the arms of someone wielding a knife is wrong. The target area is the largest body mass.
Fantail. From what I have seen so far, I am far from convinced you are ex job.