This isn't a UK rule - our customs don't have access to your (private) NHS number to even verify it, and why would they bother anyway?!? They will just hold onto the goods at the port until any VAT/customs fees have been paid, but they don't really care who pays the bill. Perhaps the seller means their country's paperwork. I think I'd just give them any random assortment of 10 digits - neither their nor our customs will be able to verify it as an NHS number anyway so it's meaningless, but their systems might need a number in the NHS format perhaps (i.e. 10 digits).