Not quite, if you say when stopped that you carry something for self defence, that item becomes a weapon as well. Also we think however it has yet to be really tested that if you carry something and the manufacturer for example on their website says it is for defence that could mean it is a weapon as well.
If you were stupid enough to say you carried something for self defence, you would probably be arrested and charged. ( although it is debatable if this would apply on your own property) anything that was labelled or sold as being for your defence, would probably be illegal , for instance having in your possession Pepper Spray has the same penalties as carrying a gun. doesn't really make any difference what a manufacturer says, it's what the Law deems it to be, and your reason for carrying it, and your demeanour.
To give you an example, about 2 years ago a Spanish woman was arrested at Heathrow Airport, she was a school teacher going home to Spain, her flight was cancelled and she was put on a later flight, she had a few drinks and eventually was deemed to be drunk, and when her flight was ready to go, she was refused entry to it, as she was inebriated, she started getting angry and aggressive, and the Police were called, she was arrested, when her bag was searched, they found in the bag a pair of scissors, she was then also charged with having a offensive weapon, even though I am sure she had no intention of using it to defend herself, this was all down to her demeanour at the time, if the Law want to get you, they will, this is why it's better if you are carrying a knife, etc, to say nothing, keep cool, and be polite.
And also to remember that although these Laws seem stringent and evasive at times, they are there to protect you.