It can’t be illegal to carry a knife as there are justifiable reasons for some to be carrying them, however how can you stop people carrying them, short of searching everyone or having huge magnets attached to lampposts?
Having made some self-defense weaponry as a product developer, the first thing i did when i went into that was to do my homework and look up what is and what is not on the banned weapons list.
There is a lot on the banned weapons list in Belgium. As an example a taser or pepper spray is not something you can legally carry in Belgium. Same goes for tear gas which otherwise in self defense would be an excellent crowd dispersal tool.
It's a pitty that weapons which would be mostly developed for self defense like pepperspray are on the banned weapons list. I suppose you could use it in a "attacking way" but the kind of weapons which i have hence developed and which i can legally carry are of the same nature.
The best self defense weapon which i know is legal and which i carry myself (and which doubles as a belt) is a bullwhip. Its just not on the banned weapons list so ... and its quite usefull if you need to defend yourself from a few people at once as a well made bullwhip will pretty much make any attacker go "nope" on the first hit. But then again i guess you could also use it in a offensive way. The "banned weapons" list is a bit dumb in a way.
Uk law is a bit less restrictive it seems:
www.westyorkshire.police.uk
Point is, knives are illegal and so are most weapons, but a creative person can always think of functional self defense tools that are outside the list and law. You wouldn't be punished for the sake of having used such a self defense tool in self defense, though the damage you cause with any weapon beyond keeping your attacker from attacking you might come to bit you. Thats why i even would prefer teargass or pepper spray over a whip, cant do this in Belgium but you can apparently in the UK. Though even in regards to those DIY is a bit tricky if you "go overboard" with it, so best option in the UK likely just is buying pepper spray and knowing to not spray to long.