It’s not clear whether from the article whether the knife itself was illegal, or whether they were illegally carrying it in public - some types of knives are always illegal (flick knives, butterfly knives etc), and there are some laws about not being able to carry knives in public without a good reason (with the exception of folding knives (penknives) 3" long or less).
Here’s the appropriate link: https://www.gov.uk/find-out-if-i-can-buy-or-carry-a-knife
One of the irritating things is that the 3" folding knife is only automatically legal if the blade cannot be locked in place. As a result I can’t carry my Leatherman in my bike pannier without having a good reason.
