The UK does not permit the carrying of *anything* for the purpose of self defense.Self defense or the defense of others is predicated (legally here) on the belief that you or the third party is at danger. The legality of using deadly force for said defense is predicated on the belief that the attacker has the ability and intent to kill or do great bodily harm. Notice I said the "belief." It's only necessary to prove that you acted as any rational person would in coming to that belief. The active display of a deadly weapon (in this case the knife) is more than enough for a rational person to believe he's in such danger (just likje the various police academies teach and just like the courts consistently rule)
We have different laws and a different culture.
I don't think this forum is an appropriate place for you to further this discussion and I can't see anything good coming from doing so.