As I said previously, the law in the UK is much more straightforward - carrying an item with a blade of 3" or more in a public place is an offence, end of story regardless of what you intend doing with it. There are only 3 acceptable defences in law (a) you need the item for work (b) it's carried for religious reasons, or (c) it's worn as part of a national costume.

Now there is room for some debate about what exactly constitutes the Scottish national costume, but generally most Scots accept that this means a shirt, jacket and tie as well as the kilt, ie fairly formally dressed.

Personally, I tend to share this view and regard the modern sgian dubh in the same light as the fancy dress sporran, a decorative item only worn when formally dressed for functions etc, and never wear it when casually attired.