The present design of the Union Flag dates from a Royal proclamation following the union of Great Britain and Ireland in 1801.

The flag combines aspects of three older national flags:

the red cross of St George for the Kingdom of England,

the white saltire of St Andrew for Scotland (which two were united in the first Union Flag),

and the red saltire of St Patrick to represent Ireland.

Read more: https://en.wikipedia.org/wiki/Union_Jack