The British Red Ensign Flag flew over the American Colonies after 1707, until the American Revolution. In 1707, Scotland’s Parliament, against the protest of many Scots, united with England and formed Great Britain. (The Robert Burns expressed the sentiment of many a Scot when he wrote the line, "We’re bought and sold for English gold, Such a Parcel of Rogues in a Nation!") The Union Flag seen in the upper corner of the Red Ensign became the official flag of Great Britain.
Many countries that were formerly part of the British Empire used the Red Ensign and added their own coats of arms or other symbol to create their own national flag. Bermuda is a good example of this. This tradition even made its way onto a state flag of our own nation: Hawaii is the only U.S. state to still have the Union Flag on its own, symbolizing its own historical background with Great Britain (Hawaii was unknown to the Western world until British Captain James Cook encountered it on one of his voyages in the Pacific).
It should be noted that the current British Union flag also has the red diagonal cross colors of Ireland, which is not seen on the flag. Northern Ireland joined with Great Britain to form the United Kingdom, the official date being January 1st of 1801.