The question is, was this not covered on your PAL course?
"...cross over into negligence?..." Negligence is what the cops will say when they're blaming you for becoming the victim of a crime and have your property stolen. A locking toolbox would result in the whole thing being stolen. Said tool box bolted to the floor is better.
One needs to remember that logic does not apply. None of these laws/regulations are meant to aid you in any way.