In this country, "the use of reasonable force" is advised.
I've been lucky
to have survived knife attacks - and never been questioned by the Police. An MA instructor once told me, that if you are questioned DON'T
mention that you used MA techniques.
Every situation is different. Level of threat is the biggest issue. I agree that if anyone pulls a knife on you, that
should be considered as a life threatening situation, therefore you must protect yourself.
It's a very grey area. I've spoken to friends who are cops and even they can't offer a definitive answer.