So it looks like the Confederate battle flag was just a variant of the Cross of St. Andrew flag (with no St. Patrick!)
But I think the Britannica is right in saying that the British flag added a (supposed) Cross of St. Patrick in 1801. And the blue and red reversed compared to the Confederate flag.
IIRC, the Union Jack is supposed to represent England (St. George’s Cross), Scotland (St. Andrew’s Cross) and Northern Ireland (St. Patrick’s Cross).......................
I have always wondered if they chose the Cross of St. Andrew because so many Scots and Scotch-Irish settled in the South, but I’ve never researched it.