Not opinion - Case Law!
Imminent threat of attack:
In Beckford v R [1988] AC 130: Lord Griffith said "A man about to be attacked does not have to wait for his assailant to strike the first blow or fire the first shot; circumstances may justify a pre-emptive strike."
Palmer v R [1971] AC 814, lord Morris made the following points:
* A person who is being attacked should not be expected to "weigh to a nicety the exact measure of his necessary defensive action".
* If the jury thought that in the heat of the moment the defendant did what he honestly and instinctively thought was necessary then that would be strong evidence that only reasonable defensive action had been taken.
* A jury will be told that the defence of self-defence will only fail if the prosecution show beyond reasonable doubt that what the accused did was not by way of self-defence.
For excessive use of force not being a defence at all, see R v Clegg [1995] 1 All ER 334 (the well known case involving a soldier at a road block).
R v Bird [1985] 1 WLR 816 - The defendant had been hit and pushed by a man. She then hit the man forgetting she was holding a glass. The trial judge directed the jury that self-defence was only available as a defence if the defendant had first shown an unwillingness to fight. The Court of Appeal quashed the defendant's conviction saying that it was unnecessary to show an unwillingness to fight and there were circumstances where a defendant might reasonably react immediately and without first retreating. It was up to a jury to decide on the facts of the case.
"Self Defence Law and Conflict Resolution " + google is your friend, as is http://www.bailii.org/databases.html