I claim no particular expertise on the issue of knife crime. But there does seem to be a somewhat-under-mentioned point here. When I was a little boy, it was very common to carry pen knives - looking in the shops at the latest Swiss Army knife with its array of interesting attachments was an innocent pleasure. And when I went to the woods to play as a teenager, I would sometimes carry a knife my sister bought me for my birthday, which I would use to carve bits of wood and help me make my fire, as I imagined myself a mediaeval woodsman or whatever other outdoor frolics I engaged in. As I understood it at the time, knives were only subject to legal control if they were more than nine inches long.
Now perhaps the world has moved on, but I wonder - I think that even in my youth, the flick knife was a known weapon of the young agile urban thug. Could there be a rural/urban divide here - that the knives perhaps even now carried for woodcraft by rural boys being harmless, whilst the knives carried by urban gang members for "self-defence" are much more problematic? It would seem a shame, to me, if we accidentally outlawed innocent outdoor fun because we had forgotten that it was possible, and that many young boys in their hoods and carrying their knives are thinking themselves Robin Hood, not Al Capone.