A sgian-dubh is tucked into one’s kilt hose on the outside of the right leg. While the blade is hidden, the handle is quite visible.
Visible or not, it’s classified as a dagger, and daggers are far too deadly to possess in public. The state prefers that I tuck a glock into my sock, like a normal American.
I thought that, traditionally, the whole point of a sgian-dubh is that it isn’t hidden. That is, you are taking it out of a hidden place and putting it somewhere visible as a sign of politeness and lack of hostility.
It’s also worth noting that UK law allows exceptions for knives worn as part of ‘national dress’ like the Sgian Dubh (although most of those you see are blunt anyway)