No such thing in this country. You're allowed to use reasonable force. That largely means the bare minimum to end the situation with the minimum of fuss. Running away basically.
You hit someone and they die because you've hit them, you've caused their death. Whether that was due to their trying to kill you or not is irrelevant in the eyes of the law.

Stupid aint it, how are you meant to accurately assess a situation when you're in a panic and need to act fast.

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No no and no