Caveat: I have not been in any kind of physical altercation since 2007. I am, however, extremely familiar with violence. I hold instructor ranks in three separate but very similar martial arts, Ninjutsu, Savate, and Muay Thai. I can't say anything about the UK or their laws, but here's my take on my experiences with the US law.
As it has been explained to me by a few lawyers, a bunch of police officers, and all of my coaches/teachers: it's not proportional force that law enforcement is looking at in cases of self defense, it's escalation of force. If I put down a violent threat, end the fight, and then immediately call the cops (and wait around to give a statement,) without going out of my way to cause extra damage to my assailant, they're not going to prosecute me.
I used to barfight frequently. I have always been told that the "appropriate measure of force," was "enough to stop someone from hurting you or others and no more." I have been in a couple of fights where a guy just refused to stop getting up until I absolutely wrecked his shit, and have never even seen the inside of a jail.
If, hypothetically, I were to kick someone, then jump on them and ground and pound them after they stopped fighting back, then I would definitely be a candidate for jail, deservedly so. Especially if I were to GnP after they were unable to defend themselves, like unconscious.
There was a case in the UK that demonstrated this, where the defendant had actually
started the fight, but the judge cautioned that being the aggressor does not disqualify the use of self-defence as a legal defence in UK law, provided it was plausible that the aggressor was not the one continuously being aggressive/disproportionate in escalation. If you go after someone looking to start a quarrel, but they pull out a knife and start trying to kill you, you may genuinely be in a position where you cannot escape or survive except by using proportionate force yourself - even though it is your fault the confrontation started to begin with. The prosecution will then point to any evidence (e.g. if you showed up trying to start a fight
and were carrying a weapon, that shows intent to escalate from the start) to try and prove you had aggressive motives to begin with. But
even showing up to start a fight with a weapon is not necesarily enough to rule out self defence:
11. ...Because the prosecution must prove his guilt, it's for the prosecution to prove that a defendant and this defendant in particular was not acting in lawful self defence. It's not for him to establish that he was. You must consider the matter of self defence in the light of the situation which the defendant honestly believed he faced.
14. What the defendant says is, he is confronted by Alex Doyle who is squaring up to him, he's throwing punches and he's returning. Then to Alex Doyle's rear are a number of other people, he and his Wythenshawe friends are out-numbered, he panics, he produces the knife and therefore it's reasonable force. All right? So you see the two stages. Was he acting and did he honestly believe he had to use reasonable force to defend himself and secondly, was the force he actually used reasonable. All right?
15. Of course the prosecution say that here it wasn't reasonable. The prosecution go further. They say it wasn't self-defence. It wasn't Alex Doyle who was the aggressor, it wasn't his friends who were the aggressor, it was the defendant who was the aggressor, he was out for trouble. That's what the prosecution say. Well, if that is the position and you are sure of it, well self-defence doesn't come into it you may think, but if you do consider that the defendant may have been acting in self defence because he honestly believed that he was under threat, you have then got to go on and ask yourself the question, was the force reasonable."
19. We would agree that the mere fact that a defendant goes somewhere in order to exact revenge from the victim does not of itself rule out the possibility that in any violence that ensues self-defence is necessarily not available as a defence. It must depend on the circumstances. It is common ground that a person only acts in self-defence if in all the circumstances he honestly believes that it is necessary for him to defend himself and if the amount of force that he uses is reasonable.
t. Royal Courts of Justice, Strand London
As in this case, wherein a chap armed with a knife started a fist fight with the unarmed deceased with revenge as his motive, but pulled out his knife and fatally wounded the deceased after the fight escalated and started involving more men, with the accused being outnumbered when he pulled out the knife. Notably the accused was still convicted of murder, but that was because the jury ruled that the use of force was totally out of proportion to what was actually happening (the guy he killed wasn't making any verbal threats, his punches weren't even connecting) nor were any of the other men getting involved in the fight actually attacking the accused. But the use of weapon, the motive and being the instigator were not per se disqualifiers for the defence
The problem is that nobody will ever be able to prove that their defense was "proportional" unless they're done in worse first. So the batter will always be at risk of being charged with battery (lol) unless the knifer knifes him a proportional amount first. The victim ends up in a moral limbo where they have to care more about the person actively assaulting them and trying to hurt them than their own safety, which is a ridiculous requirement in an emergency. And since emergency is a requirement for self-defence, we end up with a self-defeating circular definition of when you are allowed to defend yourself.
This is true, but the burden of proof is on the prosecution to prove you weren't proportional
It puts the victim at the mercy of the whims of the prosecution and judge and how keen they are on a sentence. At least in the UK they would have to go in front of a jury that might look at it with some common sense, but here in Sweden we don't have that. Which means it's entirely likely you'll be sentenced for murder for firing a hunting rifle at armed people breaking down your door in the middle of the night and shouting that they're going to kill you. Which, yes, is a real case which happened 5-10 years back.
Oh yeah we have that nonsense too where police warn people for not getting attacked first