What symbol is represented by the diagonal white cross on a blue background on the official flag of the UK?

Okay, let's break down the symbolism within the Union Jack, the flag of the United Kingdom. The Union Jack is actually a combination of flags representing different parts of the UK. The diagonal white cross on a blue background is specifically the flag of Scotland, also known as the Saltire. This cross is the cross of St. Andrew, the patron saint of Scotland. Legend says St. Andrew was crucified on a cross of this shape. When the crowns of England and Scotland were united in 1603 under James VI of Scotland (who became James I of England), there was a need to represent both countries in a single flag. That's why St. Andrew's cross became a key component. The red cross is that of St. George, the patron saint of England, and the red diagonal cross is that of St. Patrick, the patron saint of Ireland. So, while St. George and St. Patrick are also represented, the white diagonal cross on the blue background is exclusively St. Andrew.