A switch blade would be illegal. But a one handed opener would not?
Yeah, its a bit arbitrary but it doesn't really cause any serious confusion. Flick and gravity knives are illegal per se. I guess one handed openers are not because of the difficulty in definition.

Truth be told, most thugs do not walk about tooled up, for fear of stop and search. Their knives are usually hidden in a nearby hedge or something. But that's not a reason to allow them to have them in their pockets.

I don't really know why they have these amnesties, since even if you do have a "combat knife" (which are prefectly lawful to buy and own, just not advertsise as such), if you decide your knife carrying days are over why not just put it in the attic? Or indeed, in the rubbish, since that would be lawful too. Can the amnesty be nothing more than a photo opportunity to convince us the government is "doing something" about knife crime?

I'm jolly glad they took that bat'leth off the streets though, I don't know about the rest of the country but round my way you can barely step out of doors without being assaulted by teenagers carrying mild steel copies of unweildy fantasy weapons.