And, were we to quibble, the UK Union Jack is just that, the canton of the various Ensigns used in the UK. The true national Ensign, if I remember right is the "White Ensign" --which is a white field with the red cross of St George upon it. the Jack is the canton in the upper quarter of that ensign.
The UK Merchant Marine flies the Red Ensign, and all red field with the Union jack as canton. Then, there is the Blue Ensign, similar to the Red, but in blue.
The UK Union Jack (aka union Flag) came about when James VI of Scotland became James I of England and Scotland. The Union flag featured the perpendicular red on white St George's Cross on England superimposed upon the diagonal white on blue St Andrew's Cross of Scotland.
Some time in the 1800's--certainly after Trafalgar--the Union Jack was revised again, by appending the diagonal red on white Cross of St Patrick of Ireland to make the "modern' Union Flag.
Because there are three Crosses represented, that is why the red stripes of St Patrick are not centered upon the white stripes of the Cross of St Andrew, but, are offset. (It's also the only way to know if a union flag is flown upside down.)