If a defendant was, or may have been, acting in lawful self defence of himself or lawful defence of another, he is not guilty. It is for the prosecution to prove he was not so acting.
To act lawfully he must have an honest belief that he or another is being or about to be attacked, so that he needs to defend himself or the other.
‘About to be attacked’ means that the threat must be immediate.
The fact that his belief was mistaken does not deprive him of the defence, unless his mistake arose through voluntary intoxication.
If he did not have such an honest belief, that is the end of the defence.
Then comes the issue of reasonable force. The jury must ask themselves whether he used no more force than was reasonably necessary to repel the attack or threatened attack.
Consider that in the heat of the moment, a man not be able to judge exactly what force is necessary. An honest belief that he did no more than was necessary may be strong evidence that force used was reasonable.