The problem is, if the officer doesn't believe your reason is good enough, you end up having to go to court over it - which is expensive, and you may well lose

The problem is, if the officer doesn't believe your reason is good enough, you end up having to go to court over it - which is expensive, and you may well lose and catch a custodial sentence. I can't really overstate how ridiculous the UK's laws around knives are. Even tradesmen with glaringly obviously valid reasons to carry knives are recommended to lock them securely in a toolbox while not at the jobsite to avoid prosecution.