The British Red Ensign Flag (top) was the flag that flew over the American Colonies after 1707.
It was a combination of the English Red Ensign Flag (middle) and the Scottish Red Ensign (bottom). In 1707, Scotland’s Parliament, despite the protest of the Scots, united with England to become Great Britain. Of that union, Scots poet Robert Burns said "We’re bought and sold for English gold, Such a Parcel of Rogues in a Nation!" (Not that we’re here to discuss Scottish History.) At that point, Queen Anne decreed that this Union Flag be the official flag of Great Britain.
The flag itself goes back even into the 1600’s but was not much used as neither the English nor the Scots particularly liked having their own flags adulterated with the other.
If you’ll look at the flag of other countries that were formerly part of Great Britain, Australia, for example, you will see that many of these countries simply took this British Red Ensign Flag and added their own identifier.
For the American Insurgents, the Union Flag was a hated flag. It was a constant reminder that, while the Colonies were part of Great Britain, the King was not allowing them the rights they’d always had as British citizens.
One of the flags the Insurgents flew the colors of the Sons of Liberty Flag. Another one, the Taunton Flag, we’ll get to in due time.
On a side note, the state of Hawaii is the only state in the USA that still has the Union flag as part of its flag, a tribute to it’s history with Great Britain.