National Insurance number

A National Insurance number was a government-issued personal identification number, which UK citizens were generally expected to have. Moira Evans, unusually, did not have a passport, bank account or National Insurance number, as she was a Ghosty. (AUDIO: Asylum)

The Eleventh Doctor showed Craig Owens his "National Insurance number", actually flashing his psychic paper, when introducing himself to move in as Craig's flatmate. The Doctor also showed Craig his "NHS number", and a reference from the Archbishop of Canterbury. (TV: The Lodger)