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, and the second part is a three-letter code, which is the same as in the British Standard BS 6879.
BS also has Welsh language code for Welsh names of Wales and its subdivisions, where they differ from the English language names. These are not listed here.
References
"Gazetteer for the Reorganised Statistical Regions and Local Authorities in the United Kingdom", Office for National Statistics, June 1997.