Generally speaking in most states in the US, you can use a sufficient level of force to neutralize the threat.
But we also take into account ‘disparity of force’ and that allows the victim to use a higher level of force than their attackers, under certain conditons. In general, if the victim is smaller than the attacker they may use a higher level of force. If a woman or small man is under attacke by a larger man, the victim may use a higher level of force. If the victim is being attacked by more than one person they can use a higher level of force. If the victim is older or have physical handicaps or disabilities they may use a higher level of force. If the victim is surprise attacked by one or more attackers they may use a higher level of force. In most if not all cases they must fear imminent permanent injury/death.
We take this into account because in all such cases the victim is inherently at a disadvantage and a higher level of force is therefore justified as reasonable.
I never hear of this ever being considered w/r/t the british cases discussed here.
Well, it is taken into account.
The test in law is whether a reasonable person would feel you had a genuine reason to feel under threat.
A 6 foot tall 220 pound war veteran like myself would find it difficult to argue that I needed to kill an unarmed 14 year old mugger.
A 19 year old girl in the same situation could almost certainly argue that she felt she was at serious risk of being seriously injured (which would include being raped) or killed by the boy - and she would be more likely to succeed in that claim.
It is taken into account in these cases.
You may use the level of force you reasonably believed to be necessary when all the circumstances are taken into account. That includes issues of size, experience - all circumstances are relevant because they are what lead you to believe you were in danger.