Phew! That's a long description. In England and Wales the law is simple:

In protecting yourself (or another person) from perceived imminent attack, you are entitled to use reasonable force in the circumstances as you perceive them to be. What is "reasonable" is determined objectively by the tribunal of fact (jury, district judge or magistrates), i.e. what other people would think was reasonable in the circumstances in which:
(a) you believed you found yourself; and
(b) you actually found yourself.

The former should, hopefully, make sense with the latter.

Your personal characteristics will be taken into account (both physical and mental), which was not the case a few years ago (when mental characteristics were not taken into account).

And as hard as self-defence may appear to be to prove in court, it is usually clear when the force used was reasonable and also when excessive force was used. When excessive force is used, you yourself might find yourself on the end of an assault charge...