Just so I am clear on some things...... If someone is under attack, it is acceptable to fight back by whatever means necessary.

No... you always use self defence in "reasonable proportion".

If a burglar breaks into your home tonight and doesn't see you sitting there on your sofa, so you startle him and he runs away - would it be reasonable for you to shoot him in the back as he runs??? No. He damaged your property when he made entry, he probably startled you, too... but, shooting him is not proportionate to the crime he committed in this case.

Let's say our burglar has a gun pointed at you... you believe that, if you don't shoot him, he's going to shoot and kill you. Would it be reasonable to shoot him or have another member of your household do so??? Absolutely - in this case, you feel that the threat to your life is significant enough that not taking action will lead to certain death.