Would someone mind explaining about UK vs England vs Great Britain vs Britain and why Scotland, N Ireland voted to stay but they are leaving EU anyway because England voted to leave or do I have that wrong?
The United Kingdom is the whole thing: England, Wales, Scotland and Northern Ireland together (and Gibraltar). Great Britain refers to the landmass which England, Wales and Scotland is on.
The referendum was a single, straight forward in/out vote among the entire population of the United Kingdom.
However, the polling and counting of the referendum was done at a constituency level, which allowed for counts of specific regions, cities and areas to be quickly made.
Because of its sheer population in comparison to the rest of the UK, England carried the "out" vote, even though in Northern Ireland and Scotland more people voted to remain in. As the United Kingdom total vote is the one that counts, those regional preferences are in theory being discounted.
However, both Nothern Ireland and Scotland have special status under the UK Constitution, and a limited amount of devolved political power. Because of this, they may be able to make their preferences stick in those areas, despite the overall vote in the UK going against them.
Ok, so - Scotland is a country, right? England is a country, right? So, what is the UK? Also a country? Or, like, a mega-country? I guess I don't see why EU and UK membership need be mutually exclusive
Depends how you're defining a country.
Scotland does not control its own foreign policy or military. England does not have its own Parliament - there are enough English MPs in the UK Parliament to outvote MPs from every other part of the UK put together.
The UK is the country, Scotland is more akin to a US state (like, say, Texas) than it is to France. However, Scotland (like Texas) has also historically been an independent state and Scotland (unlike Texas) also has a very valid claim to being a nation - that is, a distinctive linguistic-cultural group. Scotland and England joined together to create the UK in the 1700s when England and Scotland ended up having the same King. Only in recent decades has power been ceded by the UK Parliament back to Scotland.
So, to build on the analogy: Imagine that tomorrow, America voted to withdraw from NATO. However, Texas disagreed. So Texas starts to look at how to leave America, in order to remain a member of NATO. That's probably the strongest American-centric analogy I can come up with.