Flag Thread the Third is dead. Long Live Flag Thread the Fourth!
Flag of the Incorporated Kingdoms of Greater Britain from my [not yet posted] Nation in Black TL. Essentially it's a UK where Scotland is much more represented and equal to England ("So we're not their bitch this time?" as my Scottish cohort put it). While this may sound a bit ASB, what I have planned is much more detailed and in-depth.
Short version is that a previously unknown half-scot bastard of the Black Prince of Wales happens to learn of his heritage. Skip down a few generations, and you get a man calling himself the Black Prince of Scotland trying to start a revolt in 1705 to usurp the throne from Queen Anne. The eventual compromise is a much more confederation-esque Acts of Union in which the usurper is made into an actual Prince (Full title the Prince of Alba), with full legitimate claims to the throne, leading to Scotland being made (at least culturally) a little more equal to England as it gains its own claimant to the throne. Sadly the Black Prince of Scotland would die suddenly "in his sleep" with no heirs, wife, or immediate next of kin. A distant cousin is placed as a puppet in his position and afterwards history flows rather similarly to OTL.
For a while the OTL Union Jack is used, though the blue isn't darkened, but debate arises over 1.) which cross goes on top (raised by the Scots), and 2.) whether the Scottish blue should be used (raised by the English). Eventually, we get this flag; the crosses are 'stacked' (Andrew Upper Left, George Horizontal, Andrew Upper Right, George Vertical), and black is used for the base (as the Black Prince of Scotland, credited for pushing the idea of Unification, often flew a solid black flag). For official usage, the coat of arms is placed at the center.