It's about as secret as your phone number I suppose. I.e. you probably won't write it on the side of your house but you will give it to lots of people. The NHS

It's about as secret as your phone number I suppose. I.e. you probably won't write it on the side of your house but you will give it to lots of people. The NHS having their own number is also to do with the NHS treating people who don't have NI numbers, and various historical reasons, eg the NHS started in the 1950s and I expect it was easier to just start a new number system than persuade the treasury to wheelbarrow over all the paper.