The Disunited Kingdom?

If England and Scotland get a divorce, who will get custody of Northern Ireland?
Being an American, it's hard for me to understand the exact constitutional relationship between England and Scotland.  And once you throw in Wales and the Channel Isles, not to mention Gibraltar and the Falkland Islands, or the fact the British head of state is also the head of state of several other sovereign nations, it's all just a little to confusing for my simple American mind.
But then, I know we Americans puzzle the rest of the world.  An electoral college elects our president, although we voters pretend that we do.  Texas has a constitutional right to maintain its own navy, Louisiana has a Roman law legal system, some of our states are not "states" but "commonwealths", and the Commonwealth of Puerto Rico is really a "free associated state" (estado libre asociado).  The governor of Texas--a state always eager to steal the spotlight from New York and California--can threaten Texas succession from the federal union one year, and run for president of the federal union another year.
We certainly have a diverse form of political unity.  I think the difference is that we Americans don't take these differences very seriously, and most of us don't even keep up with them.
Anyway, Scotland will vote on independence in a couple of years.  I'll have to watch Braveheart again between now and then.