As a clever reader, you are probably aware the Union Flag (or surprisingly the Union Jack as it can be known, even if not being flown at sea), comprises stylised representations of the flags or satires (diagonal crosses) of the countries in the Union.
Since Richard I declared in 1194 that England's flag would be the red St Georges Cross on a white background.
They took the Scottish flag (the St Andrew's Saltire - the diagonal cross)
The white border around the red cross is there for heraldic reasons to separate the colours.
In 1801 Great Britain enlarged to include Ireland so a new flag reflecting this was required.
The St Patrick Satire was therefore incorporated to represent Ireland.
This means that the Union Flag is not actually symmetrical.
The flag was not changed when Ireland split to form the Republic and Northern Ireland in 1921. Only Northern Ireland remains in the United Kingdom
Why does no Welsh elements appear on the Union Flag?
The United Kingdom does comprise England, Wales, Scotland and Northern Ireland.
Well, when the first Union Flag was created in 1606, Wales had been united with England since the 13th century. This meant that it's status was actually a Principality and not Kingdom.
As such, it could not be included.