The case which manisha v is referring to is R v Rose (1884) 15 Cox 540.
In this case, a young boy shot his father in his head. The father was about to launch a murderous attack on the mother. The boy intending to protect his mother shot his father in his head. The young boy acquitted on the grounds of "self-defense". This decision implies that "self-Defense" includes the "self-defense" of near and dear ones.