Pedantic: actually, there are (up to) two valid postal country names: the English one (UNITED KINGDOM) and the French one (ROYAUME-UNI, https://www.upu.int/UPU/media/upu/PostalEntitiesFiles/addres...). This is due to historical reasons: (jointly) Britain and the US (which use English) were the most advanced in postal services but the diplomatic language at the time is French.
...except for Great Britain, who were one of the first few other countries to be connected to the internet, and decided to use .uk before people realised that every country would need one, and using already-assigned ISO codes would be the best way to hand them out.
Wikipedia says "Great Britain is an island in the North Atlantic Ocean", although historically there was also the Kingdom of Great Britain.
https://en.wikipedia.org/wiki/List_of_ISO_3166_country_codes...
Edit: Also, if you've ever seen UK teams compete in international sports, all their athletes have "GBR" on their team uniforms.
ISO 3166-1:2020: Codes for the representation of names of countries and their subdivisions — Part 1: Country code
ISO 3166-2:2020: Codes for the representation of names of countries and their subdivisions — Part 2: Country subdivision code
ISO 3166-3:2020: Codes for the representation of names of countries and their subdivisions — Part 3: Code for formerly used names of countries
Maintenance Agency
ISO 3166 Maintenance Agency
c/o ISO Central Secretariat
BIBC II
Chemin de Blandonnet 8
1214 Vernier
Switzerland
Tel: +41 22 749 01 11
E-mail: customerservice@iso.org
Website: www.iso.org/mara/iso3166But is the mail in the source country sorted by people, computers programmed by people, or by the ISO committee?
Someone
BTXX XXX
GB
...would probably get there, it would technically be incorrect. Mind you...
Someone
BTXX XXX
UK
...might not get there either, if your postie was a dyed-in-the-wool republican!