Why Do UK Coins Not Say Uk?

The front of my 1920 King George V half crown says: GEORGIVS V DEI GRA: BRITT: OMN: REX

Translates to "George V by the Grace of God, King of all the Britains, Defender of the Faith, Emperor of India"

Traditional, along with the inscription. "Elizabeth II Dei Gratia Regina F D" (or some abbreviation thereof) does not, I believe, appear on the coinage of any other sovereign nation.

Back in the late medaieval / early modern period (1400-1600), most coins did not have the name of the country placed on them. They had the name of the king (or sometimes just his monogram, or coat of arms) and the King's titles - which sometimes, but not always, prominently included the name of the country the king ruled over. The British monarchy is merely one of the last countries where this old tradition is maintained. They are also one of the last to maintain the tradition of using Latin inscriptions on the coinage, and the Latin name for the country doesn't resemble the English name for it at all.

As others above have stated, prior to the reign of Elizabeth II, most British coins did indeed include the name of the country in the titles: "ANGL" for "England" prior to 1707, then "MAG BRIT" for "Great Britain", then "BRITANNIARUM" (or "BRITT") for "the Britains" (meaning the island of Great Britain proper and the surrounding islands, including Ireland) and finally "BRITANNIARUM OMNIUM", usually abbreviated to just "BR OMN", meaning "all the Britains", implying not just the British Isles themselves but everywhere else British people had colonized.

In 1952, shortly after the Queen took office, the royal titles were reviewed by a council of the prime ministers of the various Dominions of the British Commonwealth. "All the Britains" was deemed to be no longer politically correct, given that a rather large slice of Ireland formally became a Republic in 1949 and many of the "British people" the Queen ruled over now lived in independent countries. So that title was annulled and replaced with separate titles for each of the Commonwealth Realms; "Queen of Canada", "Queen of Australia", etc. Rather than replace "BR OMN" with an abbreviation of the Latinized version of the new titles for Great Britain ("BRITANNIARUM REGNORUMQUE SUORUM CETERORUM"), they simply dropped them. Perhaps they thopught that adding "BR. R.S.C." would have taken up too much space, or would simply have confused people.