The UK (or English?) law about self defence is "back to the wall", i.e. you can invoke leathal force to defend your own life when your back is against the wall, when you have no other option, and no way to escape. In other words, if you can retreat from the situation, then you must retreat.
Some places in the USA have "stand your ground" laws. These say you aren't required to retreat, that you can "stand your ground", that you can use (legally) leathal force without requiring that your back is against the wall.
Some places in the USA have "stand your ground" laws. These say you aren't required to retreat, that you can "stand your ground", that you can use (legally) leathal force without requiring that your back is against the wall.