Jonny,
if you carry anything in the name of self defence then you would be committing an offence. For example if I have a baseball bat and ball down the park with my son and we are playing and someone try's to mug me and I wack him with the bat to prevent him robbing me, no offence committed, not unless I go past the point of reasonable force and cave his head in. If I carried the baseball down the park whilst walking the dog as I've heard there are muggers about so I've got it to defend myself then it would be an offence as its then becomes an offensive weapon. I would say see the logic but there isn't any really, the law is good intentioned as it is designed to stop people having weapons to hand when they all to oftn lose their temper or become fronted by confrontational situations.
Cat
If what you say is true and someone was arrested for having a leatherman on them (and they had no other issues so to speak)then all I can say is the officers involved need a lesson in proportionality. Myself and at least 4 other officers carry a leatherman both on and off duty, the blade is great for cutting seatbelts at RTC's etc. There is an offence of having a bladed or sharply pointed article in a public place as well which covers any knife etc. This is there to combat those who use darts, craft knives and stanley blades etc in the commission of offences. The key to carrying anything that could be classed as an offensive weapon is having a "lawful and reasonable excuse" to do so. If you have a road rage incident and pull out your leatherman, swissarmy knife (or axe, or chainsaw if your a tree surgeon say) and try to intimidate or threaten the other party your lawful excuse defence goes straight out of the window. If the aformention items stayed in your pocket or vehicle and were not mentioned or seen until a copper searched your car or person, no offence has been committed.
I hope I haven't confused or lost anyone, its complicated I know, but you wanna try the police and criminal evidence act, now that is a minefield !