...is for Union.
The Act of Union in 1603 was a parliamentary act that joined the Kingdom of England and the Kingdom of Scotland under a single monarch, James I (who had previously been James VI in Scotland and was the son of Mary, Queen of Scots and Lord Darnley). The two countries nevertheless remained separate sovereign states, with their own parliaments and laws.
It was not until 1706 that a treaty was signed, advocating parliamentary union and brought into reality by the Union with Scotland act, passed by the English parliament in 1706 and the Union with England act, passed by the Scottish parliament a year later.
Since 1707, England, Wales and Scotland have thus been bound by law and named the Kingdom of Great Britain.
That union, now over 300 years old, is set to be put to the test on 18 September this year when the Scots will vote for or against independence in a national referendum. It should be noted, however, that when the Scots talk about independence, it's a very dependent sort of independence; assuming those for independence win (and right now, it's a real possibility), they hope to retain the monarch as their head of state, retain the pound sterling as their currency, become members of the EU (which is contradictory with the pound sterling thing because all new members of the EU are supposed to adopt the euro) and NATO, have a representative in the Bank of England and have England (and Wales, I guess) help them out of any financial difficulties they might incur.
I'm half Scottish (through my late mother) and half English (through my father), though I have never felt any particular ties to Scotland, quite the opposite actually.
I think the quest for independence is misguided and foolish. I think the referendum is a travesty - ALL of the UK should be able to vote on a matter that affects the whole country (the UK, I mean), but the Scottish Prime Minister doesn't really want independence (because he knows it would be a disaster for Scotland; he could never keep all the wild promises he's been making to this electorate) so he refuses to let the rest of the UK vote - if England could vote, the Scots would have their independence by about 90%!
As it stands, and given my feelings with regard to the place, I sincerely hope they do get their independence, if only to teach them a lesson. I would then hope that the EU would force them to join the euro (or not join the EU, one or the other) and that the Bank of England would stand its ground and refuse any Scottish representation (surely a concept as absurd as an Italian demanding to be allowed on the board of the Bank of France...).
The various Acts of Union were good things, positive things. James I was a good and popular king, ruling wisely over all his kingdoms. It seems a shame - the word isn't strong enough - to destroy all that now...