Britain
Great Britain is, of course, the greatest country in the world. Having said that, it is not really a country, so much as an artificialised conglomeration of several countries (perhaps our colonial cousins could make a note). Britain is comprised of England, Scotland, and Wales - there are some smaller bits added on which is why we're called 'great' - it wans't us just trying to sound pompous (although we are quite fond of doing so of course), rather that 'Great Britain' just means 'Greater Britain'. The UK includes Northern Ireland, but MG will leave NI aside for the moment - see the section on Catholics and Protestants).
The British Empire (as was) is the largest the world has ever seen - much bigger than the Roman Empire - taking in vast tracts of land, like Canada, Australia, and India. This, as Eddie Izzard has pointed out, came about by our cunning use of flags. We had a lot of flags and just went around planting them in the countries of people that didn't have flags, and if they objected, we shot them, or made slaves of them. Such is the stuff of greatness.
England
This is the biggest and best of the countries comprising Great Britain. Apart from being geographically bigger, it has lots of people too - about 50 million of the total UK pop of about 60m. 'England' is what Americans, and most of the rest of the world, mean when they talk about 'Britain'. 'The British' means the English. A British accent means the one used by BBC newsreaders (usually) - 'received pronounciation'. This is one English accent, but there are many more. There is no such thing as a 'British' accent.
The progress of civilisation can be measured by the achievements of the English. Faraday invented plastic and the electric motor. Brunel invented bridges and trains. And Shakespeare invented plays and stuff. Cromwell invented parliament, and oppression.
Scotland
The Scots pretend to hate the English, but are really just secretly jealous of our significantly better weather. (No, actually they really do hate the English). Scotland is a very cold place near the North Pole and latest reports indicate that there are still a few Scots that live there, but these reports have yet to be confirmed. The Scots have the worst diet on the face of the earth - deep-frying, as they do, Mars bars. It is just possible to get a Scot to eat a piece of fruit, if you deep-fry it, but nobody has been brave enough to try. Scotland is so cold that, historically, they have had to keep inventing stuff just to keep themselves warm. Where we have had Faraday, they produced McAdam who invented roads. Where we had Brunel, they had John Logie Baird, who invented the telly. But where we produced Shakespeare, all they could manage was Rabbi Burns, although what a Jewish guy was doing up there we don't know. They also were responsible for Alexander Graham Bell, who, despite inventing the telephone could not spell his middle name in a proper Scottish way. But surely Scotland's proudest moment was to produce the greatest footballer ever - Kenny Dalglish. If you see some of the goals he scored, it becomes apparent very quickly that his achievements, his artistry, his invention, far outstrip those of Baird, Bell and Co. Oh, they did produce a pretty decent philosopher - David Hume.
Wales
It has been rumoured that there is a small country just to the left of England, called Wales. At least, this country used to exist, but sadly no longer does - otherwise it would surely have been put on a map of Europe, and it wasn't, so it doesn't: (https://news.bbc.co.uk/1/hi/wales/3715512.stm).
This is a great shame, as Wales is often used to measure bits of land - wanna know how big the Jasper National Park is - "about the size of Wales", or how big my farm is in Wisconsin - you've guessed it: "about the size of Wales".
The Welsh hate the English even more than the Scots do. This is because the Welsh got beaten up and conquered by the English, and they can't get over it. Also, unlike the Scots, there have been no famous Welsh people ever.