ISO 3166-2:GB is an International Organization for Standardization (ISO) coding system that gives geocodes to administrative divisions of the United Kingdom:
The Channel Islands and the Isle of Man are not part of the UK, but ISO offers codes for them under "GB-".
The first part of each code is the ISO 3166-1 code GB for the United Kingdom. The second part is a three-letter code which is the same as in the British Standard BS 6879.