What constitutes the United Kingdom is probably not what many would think. For a start, it is important to distinguish between the term "United Kingdom", and the term "Great Britain". The two are not the same. Great Britain does not include Northern Ireland, and indeed the official title of the United Kingdom is "United Kingdom of Great Britain and Northern Ireland." What may come as a bigger surprise to many is that the Isle Of Man, lying as it does between Wales and Northern Ireland, is not part of either Great Britain or the United Kingdom.
The term "Great Britain" relates specifically to one island. This is the island that is made up entirely of England, Scotland and Wales. Any other territory is not part of Great Britain, even though it may be part of the United Kingdom, and even governed by the mainland. England is the largest constituent country of the United Kingdom, and is home to 50 million of the 60 million people who inhabit the entire Kingdom. England was a country in its own right until 1707 when The Kingdom of England, which already included Wales, merged with the Kingdom of Scotland.
There are still some people in the world who refer to the whole of the United Kingdom as "England", something which the Scottish for one are not exactly pleased with. Scotland was an independent country until 1707, when it merged with England to form the Kingdom of Great Britain. There were widespread protests, and ever since that day there has been a hard core of Scottish Nationalists trying to get status as an independent nation again. Feelings against the English are especially strong in the Highland areas, the areas in the far North of Scotland.
The Principality of Wales has never been an autonomous state, although it developed its own distinctive culture after the Romans withdrew in the 5th Century. Wales was formally conquered by England in the 1500s, with Welsh law being completely abolished. This was in response to the efforts of Welsh nationalist Owain Glyndwr, who had, for a brief time, led a rebellion which saw several independent principalities become united as an independent kingdom. Modern Wales is very diverse, with large tracts of land in the centre of the country extremely rural, and used for livestock farming, contrasting with the heavy industry of South Wales.