This is the Real Difference Between Great Britain and the United Kingdom

Despite popular belief, Great Britain and the United Kingdom cannot be used interchangeably.
There is no country known as “America.” America does exist, however, in a strictly geographical sense. For residents of the United States, however, America is seemingly synonymous with the name of their country of residence. But, strictly speaking, “America” or “the Americas” is a geographic term, while “The United States” or “The United States of America” is a political term.
Now that the stateside parallel is established, it’s time head across the pond to address the usages of Great Britain vs. the UK, or United Kingdom. The difference between the two, at least in the modern day, is also based in political and geographic naming.
Great Britain is another name for the island of Britain, which, politically, is composed of Wales, Scotland, and England. When used in a political sense, it extends the island’s boundaries a bit to include some outlying islands, such as Anglesey and the Isle of Wight.
The United Kingdom, however, does not cut both ways, as it is a purely political term. The UK is an independent country composed of Scotland, Wales, Northern Ireland, and England. All of Great Britain is within the United Kingdom, but the United Kingdom is not 100 percent composed of Great Britain. Make sense?
[Source: Encyclopedia Britannica]
Heading across the pond yourself? Be sure to check out the top 50 London attractions.