I learnt my NHS number about 50 years ago, thinking it was mine for life, but this turned out not to be the case. When a different doctor's surgery asked me for my NHS number, I quoted that number and was told it was wrong, and I was given a completely different number. More recently, when I had a hospital appointment I discovered I had yet another number. I haven't bothered to learn that one.
I do know my National Insurance number, which hasn't changed since I acquired it in my teens.