Details on my right of self-defense and public fighting?

The law is pretty clear on self defense. You are entitled to use reasonable force to defend yourself or others from attack. If your dad unloads on one of these punks and hurts him badly the police would have to look at whether or not what he did is reasonable. If for example he was attacking your sister and your dad intervened to stop it the degree of force used would probably be greater than a retalitory strike where the kid accidentally backed into your sister. You have to look at intent of the ailants, and the danger they posed to others. If the police arrive and subdue them they are supposed to use reasonable force - obviously if they shoot them out of hand that's not legal - same with your dad or any other martial artist - they should know how much force they can use. Too much is unacceptable even if these were punks who deserve a good kicking.