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.
Iain.