Curious as to why the UK has one code instead of one for each country?
Why isn’t Scotland SC, England EN etc?
This question comes about because I recently noticed the GF range from Postal love has the United Kingdom as one of it’s countries with London as its capital, I found this odd as the UK isn’t a country. I wonder now if they went down the code list on Postcrossing?
Technically the country is United Kingdom of Great Britain and Northern Ireland, so UK. Great Britain is the main island, including England, Scotland and Wales but excluding Northern Ireland.
As @yudi mentions ISO code for the United Kingdom is GB and GBR for the three letter one.
Thank you, I wasn’t aware of this list
It’s a sovereign state containing several individual countries.
I wasn’t aware of the list that PC followed. 
I recently saw a pretty cool postcard about this.
It’s in German, “Vereintes Königreich” = “United Kingdom”, “Großbritannien” = “Great Britain”, “Britische Inseln” = “British Isles”
UK is a country.
I was going to write a long post about the connotations of ‘country’, ‘nation’, and ‘sovereign state’, but decided it was too pedantic. The short version is that in some sense the word ‘country’ often implies a geographic area where an ethnic group lives and which often but not always has its own government, and a state is an area which is governed by a particular entity, and they aren’t necessarily the same thing. England is a country, Bengal is a country, Russia is a country The United Kingdom is a state which includes the country of England, the Soviet Union was a state which included the country of Russia, and India and Bangladesh are states which include portions of Bengal. Since typically states are the entities which control post offices, it seems easy to create codes based on the names of states rather than countries.
This still feels a bit pedantic but I had a glass of wine so I’ll click the post button anyway. 
To some extent I am glossing over stuff but I think this hits some of the reasoning on why they may have chosen to go with GB instead of separate codes for the different parts of the UK.
Your explanation is very satisfying for my OCD brain that wants everything to align! I always thought that country and state more or less mean the same. I guess that’s due to me not being a native speaker. 
I would definitely say the words get used very similarly in casual usage because they are about closely related topics but to the extent there’s a difference in meaning, it’s basically what I tried to describe.
on Postcard United we can choose if we have GBR or ENG.
Scotland and Wales have their own if they wish to use it.
it is strange we use GB.
Switzerland uses CH which isn`t in the spelling now.
I think CH come from the latin name: Confoederatio Helvetica
And avoids giving preference to one of the four national languages over the others. Likewise the stamps say “Helvetia” on them.
And Croatia HR. No way near the English spelling either but Croatia is Hrvatska in Croatian and therefore the ISO code HR.
Or DE for Germany. In German it’s Deutschland. So DE fits.
Or EE for Estonia. In Estonian it’s Eesti. So EE fits.
Or IS for Iceland. In Icelandic it’s Ísland. So IS fits.
In conclusion: ISO codes are not based on the English spelling. Nevertheless they often match the English names too.
Another favourite of mine is Algeria: DZ.
DZ comes from the Arabic name, which can reasonably be written in Latin characters as Al-djaza’ir.
I often put Deutschland when sending to Germany.
