What does the blue on the UK flag mean?
UK means United Kingdom (aka Great Britain). UK consists of 4
nations, England, Scotland, Wales and Northern Ireland.
UK flag, known as the Union Jack, consists of 3 national flags
overlaid. England's flag, red cross (vertical/horizontal) on a
white background. Scotland's flag, white diagonal cross on blue
background and N. Ireland's flag, red diagonal cross on white
background.
The three national flags are named for the patron saints of each
nation.
England - St. George
Scotland - St. Andrew
N. Ireland - St. Patrick
Patron saint of Wales is St. David but the welsh flag isn't
represented in the Union Jack.