It's a topical subject here in the UK. Some parents of stabbing victims have just started campaigning for a mandatory 6-month prison sentence for carrying a blade shorter than 3 inches. 5 years for longer blades. As reported by, eg,
The Independent, another child dies in a knife attack every 2 weeks.
These people are blinded by grief, and cannot see that knifes have legitimate uses. There is a more researched account at
Channel Four , which says that "the children" are usually aged 17 to 19. The politicians themselves seem to understand that mandatory sentences are a bad idea. Current UK laws do allow you to carry a knife, of any length, if you have a reasonable need - eg for wood craft. It provides scope for courts to use common sense. (Although it was a court which decided that a folding knife is not a folding knife if it can be locked open - which I don't think is what the statute intends. Perhaps if the law is reviewed this can get clarified explicitly in a reasonable way.)
I completely agree about demeaner etc. Also, in the UK we rarely get searched (I've never been), so unless you are the kind of person who gets into knife-fights there's unlikely to be a problem in practice. If I wanted to carry a locking blade, I would, and I'd argue the toss in court if necessary.