Yeah, he was stupid.
When they say you need a reason, you need a reason that you can prove.
A good reason is largely taken as "for your employment". If you are following a hobby or leisure activity then you pretty much have no proof and you will get a caution.
That depends...
It's not exclusive to "for employment", but the good reason needs to apply
at that time.
If you're sat next to a lake/river, rod in the water, gutting a fish, you have a good reason
at that time and you almost certainly won't get in trouble for having a knife (assuming it's not on the banned list).
If you're on a beach and you're battoning some kindling to light a campfire, you have a good reason
at that time and you almost certainly won't get in trouble for having a knife (assuming you are allowed to light fires on the beach).
If you're travelling to/from a lake/river and you have a gutting knife in your tackle box tucked away in your backpack or in the boot of your car, you almost certainly won't get in trouble for it.
If you're walking from your car to the beach with a bundle of firewood and a picnic basket with some sausages/marshmallows/etc. a knife and some fire-lighting tools in it, you almost certainly won't get in trouble for it.
If you're walking to/from the lake/river/beach with that same knife on your belt, you do not have a good reason to have the knife on your belt
at the time, and it should be stored securely.
That still applies to tools of the trade. A chef with a couple of kitchen knives in a roll, safely secured in a backpack on the tube on the way to a shift is going to be fine. A chef with a kitchen knife tucked into their belt on the tube on the way to a night out on the other hand...
Edit: of course the safest* option is to limit your carry to a non-locking folding knife with <3" blade, but if you can prove a good reason (and hobbies are a good reason, just significantly harder to prove) then it's not quite black and white
* safest in terms of legality, safety of your fingers may be another matter!
