UK is short for the United Kingdom of Great Britain and Northern Ireland. The UK is a country consisting of different parts that used to be seperate countries. Some people count these as still being different countries because some laws are different (and also a few people just think 'they have different names so they're different countries') The UK has one currency, one government, and one 'normal' language (the majority of people don't speak Welsh, Scots, Gaelic, or anything, and those who do all speak English as well). The UK is a part of the continent Europe. Some people talk about 'Europe' as if the UK isn't part of it. What they actually mean is mainland Europe.
(In alphabetical order because we shouldn't assume any parts are more important than others)
In the British Isles:
Republic of Ireland
some other little islands
Great Britain (shortened to Britain):
maybe some of the little islands?
United Kingdom of Great Britain and Northern Ireland (UK):
probably some of the little islands
UK, United Kingdom, and United Kingdom of Great Britain and Northern Ireland are all interchangable. Britain and Great Britain are interchangable. The Republic of Ireland and ROI are interchangable. Ireland is as well but can also be used to mean the piece of land containing Northern Ireland and the Republic of Ireland. Apart from these, all the other words mean different things. UK, Britain, and England do NOT mean the same thing.
There should not be a stereotype for British or English people or people from the UK (come to think of it, there shouldn't be any stereotypes at all anywhere).
*People from the UK are not all posh.
*Like in any country, there are lots of different accents. So if you hear weird accents on TV, don't assume that's how everone sounds.
*Not everyone likes tea.
*Not everyone likes the Royal family.
*Some people like Americans, some hate them, and some really don't care.
*Most young people find the idea of eating oatmeal soaked in blood (black pudding) completely disgusting.
*There are some Scottish people out there who think eating yucky bits of sheep (haggis) is disgusting.
*Not everyone likes football. Football is the same as soccer is in America, and rugby is either the same as or just a similar idea to American Football (because originally there was AsSOCCiation FOOTBALL, and RUGBY FOOTBALL. We just chose to shorten them in different ways.)
I'm not going to list everything, because I can't be bothered, and anyway you get the picture.