The Union Jack is the flag of the United Kingdom. It is actually a clever combination of the national flags of England, Scotland, and Ireland. With the recent surge in Scottish nationalism and talk of Scottish "devolution," the cross of St. Andrew could eventually be removed from the Union Jack.
This presents another interesting British constitutional question. Because the current monarch of England actually owes the English crown to the fact that they are, historically, first the monarch of Scotland (due to an interesting twist of succession 200 years ago); if Scotland removes itself from the no-longer-United Kingdom and remains a monarchy, how will the, supposedly inseparable but then separate, crowns of Scotland and England be worn?