We do prefix 'the' to 'UK' because 'UK' is shorthand for 'the United Kingdom [of Great Britain and Northern Ireland]'. As this is an official country name we can use the definite article, e.g. 'the United States [of America', 'the People's Republic of China', 'the Kingdom of Thailand' and so on. read more
The definite article the is the most frequent word in English. We use the definite article in front of a noun when we believe the hearer/reader knows exactly what we are referring to. • because there is only one: The Pope is visiting Russia. The moon is very bright tonight. The Shah of Iran was deposed in 1979. read more
We've established that Great Britain is the name of the island containing England, Scotland, and Wales, but "Britain" is really an archaic term for the Roman territory of Britannia, also called Britannia Major to distinguish it from Britannia Minor (the area of France now called Brittany). read more