As a regular wearer of full Scottish attire in London, I asked the Police what the ruling was for wearing my sgian dubh. They said as long as I was in full National dress, that it was ok. I travel through some rather risky areas in north London and even though police have been scanning at tube stations through an arch, they have never asked me to go through, even though they have clearly seen me wearing my sgian dubh in my sock. However I do tend to wear a fake unless I am in full formal Argyll, Crail or Prince Charlie.