" A police officer must believe an item is going to be or is being used to harm for it to be an offensive weapon "
If you're carrying a 3 foot Katana in a public place with absolutely no intention of using it..or causing harm..you'll be arrested. It is an offensive weapon, whether you intend using it or not.. A 3 foot Katana or a 4 inch sheath knife it doesn't matter..they are offensive weapons and unless you can satisfy a cop as to why you are carrying it (them) for a legitimate purpose, you commit an offence. "I don't intend harming anyone officer, I always walk up the high street/ across the park/ over farmer Giles field with this"...not good enough.
A 4" sheaf knife isn't an 'offensive weapon'. This has been done to death on this forum and others.
You also won't automatically get arrested for carrying around swords, spears, axes, etc.
I have several friends who take part in European Historical Combat events and reinactment. They often travel by train, carrying swords, polearms etc. Never once been arrested.
J***, who is irish, was stopped at Birmingham New Street station by two police officers, wanting to know what he was carrying. He had a rucksack containing a sword, hilt sticking out of the top, and a couple of axes. In his hand was an 8foot polearm. He described what they were and where he was going, and the police sent him on his way.
Now if a young man with an Irish accent, a shaved head and doc martins laced up to the knee can get away with it, anyone can.
Justifiable reason for carrying, reasonable attitude. That's all you need.