Aha, az.

GB [Great Britain - as opposed to "UK", which encompasses Northern Ireland] Before 1922 it was used for United Kingdom of Great Britain and Ireland

GBA Alderney Great Britain - Alderney

GBG Guernsey Great Britain - Guernsey GBJ

Jersey Great Britain - Jersey GBM Isle of Man Great Britain - Man

GBZ Gibraltar Great Britain - Gibraltar [Z was assigned as G was already used for Guernsey]

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