There's not a hair on my head that's not grey but at least it's still there
That's not true. You don't have a single grey hair. Hair either has melanin and it's coloured, or it doesn't and it's white. Grey hair is a mix of coloured and white hair that looks grey in total from a distance.
My hair is completely white and has been since I was 16 and a bit.
Yup, got it pictured in my head....
I can’t understand why it spreads upwards.
First pubes, then chest, then nose hair, then eyebrows. Now they are working their way up my head, starting with my sideburns.
That's not my case. Definitely started on my head and the bits in the middle are still... oh, I'm not discussing that here!
But now I come to think of it, my leg hair has disappeared. Maybe it's white, and invisible against my anglo-celtic skin. But that definitely happened after my head.