There are several edge cases like this in the UK.
It's obviously not legal for say, me, a middle aged man, to possess a photograph of some 17 year old girl with her tits out. Right? Except well... wait no, because what if I have a photo of my middle aged wife shortly after we first met, so the photo shows a 17 year old girl with her tits out, but the person is my wife, who is like "Yeah, remember when I had long hair? Also, I wish my tits still looked like that". So clearly that is OK after all, it's legal.
And then we have a huge row, she divorces me, now that photo is illegal after all, because I'm definitely not allowed to have photos of under-age girls with their tits out, and now the photo isn't of my wife... not any more.
Knife laws similarly have weird edge cases. 12" long sharp blade? Crime. In a Kebab shop to make delicious kebabs? Legal. I took it with me to the pub after work? Crime; Walking down the street with an ordinary Swiss Army Knife (oversize)? Crime. Tiny version of that knife? Legal. Sword, like an actual medieval sword? Crime. But I need it for this mock battle we're staging? Still a crime. No swords. Use a fake sword which can't hurt anybody or go to jail.
Edited: The "original" Swiss Army Knife is barely short enough that it's always legal, but some oversize variants are not. Like that Kebab knife you can have a lawful reason you needed to carry the knife regardless of size but I hope your reason make sense ("Self defence" is never a lawful reason to carry weapons in the UK)