In all the years I have been married to a lawyer I have learned 2 things, never ever discuss anything legal with wifey and the law and logic have nothing in common, no intersection whatsoever. On the third hand I do not understand the UK legal system so I really do not have an opinion on that. If used for stabbing A would make sense, if used for slashing none of those (it does not make a difference).
If it was determined on the ability to stab, then all of those measurements wouldn't matter, as even a blade less then 3 inches could stab and kill someone, I know they had to draw the line somewhere, but the law does state, a knife or sharp implement, so in theory the length of the knife wouldn't matter or how big your pen, screwdriver, chisel, etc, etc was, the determinating factor would be why you have it, and what you intended to do with it..
Saying that, a good defense would be the way the Law is written, the following is copied from the Govt website, and you can see just how misleading it is.
It’s illegal to:
- sell a knife to anyone under 18, unless it has a folding blade 3 inches long (7.62 cm) or less
- carry a knife in public without good reason, unless it has a folding blade with a cutting edge 3 inches long or less
- carry, buy or sell any type of banned knife
- use any knife in a threatening way (even a legal knife)
which in truth puts you in a precarious position if stopped and searched, and probably your attitude would be the important factor.