Why the flag of United Kingdom has crosess?

The flag of the United Kingdom, commonly known as the Union Jack, does not have crosses but two diagonal red lines representing the crosses as used by England and Ireland. The Union Jack is a combination of the flags of England, Scotland, and Northern Ireland.

* The red cross on a white background represents England, it is associated with St George, the patron saint of England. The cross was the flag of England prior to the Act of Union in 1707.

* The white diagonal cross on a blue background represents Scotland. It is associated with St Andrew, the patron saint of Scotland. It is the flag of Scotland and was incorporated into the Union Jack in 1707.

* The red diagonal cross on a white background superimposed on the St Andrew's cross represents Northern Ireland. It is the flag of Northern Ireland and was incorporated into the Union Jack in 1801.

Wales is not represented in the Union Jack, despite being part of the United Kingdom. This is because Wales was already incorporated into the Kingdom of England prior to the creation of the Union Jack.

So the Union Jack is not a flag of England but instead of the whole United Kingdom.