"When people say England, they sometimes mean Great Britain,

sometimes the United Kingdom, sometimes the British Isles -
 but never England."

"How to be an Alien" by George Mikes

"England" is sometimes, wrongly, used in reference to the whole United Kingdom, the entire island of Great Britain (or simply Britain), or indeed the British Isles. This is not only incorrect but can cause offence to people from other parts of the UK.

England, Scotland, Wales, Great Britain refer to different parts of the UK. The British Isles includes many islands not even part of the UK.

The name United Kingdom refers to the union of  four separate countries: England, Scotland, Wales and Ireland (though most of Ireland is now independent, only Northern Ireland remains part of the UK).

The UK's full and official name is the "United Kingdom of Great Britain and Northern Ireland".

Britain

Britain comprises only England and Wales.The name Britain goes back to Roman times when they called England and Wales "Britannia" The Roman province of Britannia only covered the areas of modern England and Wales.

Great Britain

Great Britain comprises only England, Scotland and Wales. Great Britain is an island lying off the western coast of Europe, comprising the main territory of the United Kingdom. Great Britain is the official name given to the two kingdoms of England and Scotland, and the principality of Wales.

British Isles

The British Isles - made up of several islands. Great Britain is the largest one. 

"A group of islands in W Europe, consisting of Great Britain, Ireland, the Isle of Man, Orkney, the Shetland Islands, the Channel Islands belonging to Great Britain, and the islands adjacent to these" 
 Collins Dictionary 2014

Many people think that 'English' is the same as 'British'. It is not! 

 People who are English are from the country of England. On the other hand, British people are people who live in Great Britain (Britain) and the UK.

England is only one of the three countries in Britain (Scotland, England and Wales). The Scots and the Welsh sometimes get angry when they are referred to as 'English'. They do not live in England and they have their own parliaments, so why should they take their identity from England? They have their own identity.

People born in England are called English or British and can say that they live in England, Britain and/or the UK. Most people in England tend to say they are British rather than English.

People born in Scotland are called Scottish or British and can say that they live in Scotland, Britain and/or the UK. 
 Most people in Scotland will say they are Scottish rather than British.

People born in Wales are called Welsh or British and can say that they live in Wales, Britain and/or the UK. Most people in Wales will say they are Welsh rather than British.

People born in Northern Ireland are called Irish or British and can say that they live in Northern Ireland, Britain and/or the UK. Most people in Northern Ireland will say they are Irish rather than British.

In Britain, the main language is English (British English). In Britain, every part of the country has its own way of speaking English. People in Yorkshire sound very different to people in Surrey; a Somerset accent is very different from any Scottish accent and it's hard to believe that people from Birmingham  speak the same language as those from Cornwall. Most people in Britain can guess where someone comes from by the way they speak, either by their accent or by the words they use.

People in Wales speak a completely different language. About 25% of the people there still speak in their native Celtic tongue called Welsh.

Shwmae? in Welsh means How are you?

Hoffet ti ddiod? means Would you like a drink?

In some regions of Scotland, Gaelic is used as a first language (particularly in some areas of the Highlands and the Western Isles). All over Scotland, the accent varies, some words are different but overall it is not too difficult to understand.