The gravity knife statute requires that the knife lock open. Slip joints and other nonlocking knives (opi #4, higo-no-kami,Boker 42 series ball joints) s/b legal.

Why did you tell me about Higo-no-kami??? Now I'm going to buy it!

Can't find opi #4 and Boker 42. I found Boker V-42, which doesn't look legal.

Be warned that NYPD has arrested people for non-locking blades.

In the UK it's illegal to carry a weapon. Anything made as a weapon; eg a knuckleduster, adopted to be a weapon; eg. a stone in a sock, or intended to be a weapon is illegal.
To prove you intended to use it as a weapon police are trained to question you along the lines of "if an attacker came at you; would you defend yourself with this?"

If you ask them "well would you?" and they answer yes. You can make a citizens arrest if they are holding it, they just admitted committing a crime :-).
qjs