The BBC has an article on other designs for the British union jack (BBC website usage is to keep it lower case) from 1604. Plenty of rich history here, for a flag which actually has no official stature with the government of the U.K. Naturally the comments which follow are just as interesting as the article itself. It's only a jack when flown from the jack staff of a Royal Navy ship. Where is Wales represented on this flag? Ireland didn't exactly join the Union. Lots of scots refuse to fly it.
In my science fiction writing, I've a 29th century British colony on one planet and had to come up with a modification of the union jack to fit their feisty independence -- following the model of superimposing new attributes onto the existing flag. And now I find that I've done as "good" a job as they did originally. (grin) Believable world building is in the details, whether you include them in the stories or not. (double-grin)