Basically the definition in law is that an offensive weapon is anything made/adapted/intended to cause injury to or incapacitate a person, and "a person who, without lawful authority or reasonable excuse, THE PROOF OF WHICH SHALL LIE WITH THEM, has with them in a public place, an offensive weapon"
Basically the law means the police have to prove you had it on you (i.e. Their statamets say "I searched the person and found a knife"), and it's down to you to prove the reasonable excuse/lawful authority. I would say your scenario would be fine - you're on your way to a course, which is specified on the letter. The letter explains the nature of the course and I would imagine that the other items you have on you and/or the way you are dressed would also back it up. That and of course the letter would probably have a contact number for the company. All police officers have means of making phonecalls and so the situation can be dealt with within minutes.
So long as you are in a place to which the public have access (whether you have to pay to be there or not - i.e. a cinema for example), or are in a place that you do not have permission to be, you can be arrested for it. It entirely depends on where you are, why, what you are doing and what you have on you. And of course, it goes without saying, if asked if you have anything on you, tell them where it is on your person. DON'T pull out a parang Croc Dundee style and say "yes mate, I'd got this"!
again, and with most things in life, common sense is paramount, but it does depend on how you react to the police stopping you and how the conversation goes.
I know people who will arrest people straight away and then get an explanation, de-arresting where necessary. To be honest, if someone is fine with me and the items are nowhere near them while I'm questioning them, I'll get to the bottom of it and deal with it as necessary. Safety first and all that jazz...
Basically the law means the police have to prove you had it on you (i.e. Their statamets say "I searched the person and found a knife"), and it's down to you to prove the reasonable excuse/lawful authority. I would say your scenario would be fine - you're on your way to a course, which is specified on the letter. The letter explains the nature of the course and I would imagine that the other items you have on you and/or the way you are dressed would also back it up. That and of course the letter would probably have a contact number for the company. All police officers have means of making phonecalls and so the situation can be dealt with within minutes.
So long as you are in a place to which the public have access (whether you have to pay to be there or not - i.e. a cinema for example), or are in a place that you do not have permission to be, you can be arrested for it. It entirely depends on where you are, why, what you are doing and what you have on you. And of course, it goes without saying, if asked if you have anything on you, tell them where it is on your person. DON'T pull out a parang Croc Dundee style and say "yes mate, I'd got this"!

I know people who will arrest people straight away and then get an explanation, de-arresting where necessary. To be honest, if someone is fine with me and the items are nowhere near them while I'm questioning them, I'll get to the bottom of it and deal with it as necessary. Safety first and all that jazz...