NI number. The personal tax ID certificate issued by the British Inland Revenue Department is called "National Insurance Number" (National Insurance Number), referred to as "NI number". It is an identity certificate that proves that you live and work in the UK and pay taxes for this. There is no uniform format and style for NI number certificates issued by the British tax office. NI number documents usually contain information such as personal name, date of birth, NI number and photo. People of different age groups and different groups hold slightly different NI number documents.