To put this approach into the context of the current legal position (in the UK) here are some extracts from a recent article from The Times (12th December 1995) on the subject. 

‘The law says that a person charged with a crime may plead that he acted as he did to protect himself, or his property or others from attack, or to prevent a crime. This, strictly speaking, is not a "defence", but a justification for the defensive conduct which, if successfully pleaded, makes the conduct lawful.

It is clear that a person may make a pre-emptive strike. "A man about to be attacked," said Lord Griffiths in one case, "does not have to wait for his assailant to strike the first blow or fire the first shot." Neither does he, or she, have to retreat before using force. The force must be "necessary" but it is left to juries to decide if an assailant could have been avoided rather than attacked.

Force used against an attacker must be "reasonable in the circumstances", and it is relation to this phrase that the greatest confusion has arisen. One line of judicial pronouncements, concluding recently in the House of Lords decision in the case of Private Lee Clegg, states that the test of whether the force was reasonable is objective. Thus if the prosecution shows that, in fact, the force used was excessive, then the defence fails. There is, though, Court of Appeal authority for saying that the decisive factor is whether the force used was reasonable according to the defendant's perception of events. Either way, however, as Lord Morris said in a case in 1971, "a person cannot weigh to a nicety the exact measure of his necessary defensive action".’

‘The Law Commission has recently proposed a test which blends subjective and objective elements to discover whether the force used by someone was reasonable. The key question would be whether the violence was "reasonable in the circumstances he believed them to be"’

As you can see the law is still somewhat vague on this matter but in our opinion if you find yourself in a situation that you believe is about to become imminently violent, striking first to finish and get away is not only far less dangerous to both parties than a prolonged brawl in which both could be seriously injured but as explained in the extracts above, such action could be considered lawful. It must be pointed out that if you get the chance to get away without conflict then grab it with both hands, no amount of training can guarantee the outcome but bear in mind that your prime concern must be your own personal safety and the safety of those with you, although you may find yourself having to justify your actions in a court of law.