To clarify on UK law:
There are three categories of weapons offenses as I understand them, although the categories are really an amalgamation of many laws:
1) Owning a banned weapon.
This includes handguns, knuckledusters, concealed weapons and so on. I believe concealed weapons includes lock-knives, balisong, sword-sticks and spring-loaded knives. You can't have these in your house.
2) Misuse of a restricted/licensed weapon.
This includes things like shortening a shotgun, having an unlicensed rifle etc.
3) Carrying an offensive weapon
This applies quite a lot to martial artists, as some of us carry training weapons to practice. To be carrying an offensive weapon it must be:
a) Created, adapted or intended to be used as an offensive weapon
b) To hand
c) In a public place
Thus you can have a 3 foot sword in your house or locked up in the boot of your car, but not on the passenger seat or maybe not in a soft bag, though 'to hand' is a slightly loose concept.
I'm not sure if weapons designed exclusively for training are seen as designed as offensive weapons, I'd think not. However if you are not carrying them to practice, the court will probably assume that you intended to use the practice weapon as an offensive weapon. Intent is hard to prove, but that won't stop you getting arrested.
A rule of thumb is that if it isn't clear that you aren't intent on using an offensive looking item you are carrying as a weapon, then you may be arrested. You'll probably get cautioned and a record too. Not good.
Something like a kabutan keychain will get you arrested everytime, since it was created as an offensive weapon. If your keys are to hand, which is likely, and you are in a public place you are in trouble.
When I worked in a MA shop, we used to wrap up swords and such, which were created as weapons, as much as possible, to prevent them being 'to hand'.
Smashing the end off of a bottle is adapting an item to be an offensive weapon.
Carrying an offensive weapon for the purposes of defence is still carrying an offensive weapon.
A shield or helmet might count as a defensive weapon, I don't know, but offensive seems to have the meaning 'could be used in offense'. So intending to use an offensive weapon seems to mean 'intending to use a weapon which could be used for offense'.
I don't know about weapons of opportunity, I've never found the relative section in the various acts of law pertaining to weapons, but common sense dictates that the intent of the law doesn't seem to extend to instantaneously picking up a weapon, or using an item as a weapon. I'd like to be sure though. If anyone has any suggestions to how I might find out, I'm happy to know.