Hypothetically speaking, unless you'd argue that a person has to physically attack you (meaning punch, kick, stab, shoot, etc.) before you can assume they're trying to hurt you, there has to be a line where a person is considered a threat before they've actually harmed you.
Don't disagree, and UK law as I understand it doesn't require you to take the first punch.
This whole discussion is hypothetical based on probably reading too much into one media report, which btw won't have mentioned charges against the burglar because that isn't the story (and the link now says the burglar is dead). There isn't much we can tell, but the investigation will be to determine whether or not what happened here is anything like your scenario...
"Obviously, if the threat has been neutralized (ie: they're lying face down and you have a gun on them waiting for the cops to arrive), you're no longer justified to use lethal force.
...which is entirely possible. There's nothing in this article to confirm or deny the possibility that the act of throwing him out the window wasn't akin to shooting your burglar who is lying face down.
Nowhere civilised that I am aware of do you have total and utter rights to do whatsoever you wish to anyone you find on your property. Anything that smacks of execution is going to be investigated, to think otherwise is fantasy.
I think there is some misunderstanding here. iirc, in the UK, a person must be "arrested" before the cops can question them. It doesn't necessarily mean that charges are pending, as it usually does here.
I'm not a legal expert, but an arrest doesn't mean charges are imminent or likely as you say. Questioning will have been carried out under caution.