The Union flag represents the whole UK. The red and white is specifically England, so they're not really interchangeable. Don't use the red/white to represent the whole UK or it'll likely piss off the Scots, Welsh and Northern Irish. The Union flag is/was made by combining the England and Scotland (white diagonal cross on blue) flags with a lesser-known one of Northern Ireland (red diagonal cross on white). We apparently forgot the Welsh, but their flag is a dragon and is very different from the others.
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The Union flag represents the whole UK. The red and white is specifically England, so they're not really interchangeable. Don't use the red/white to represent the whole UK or it'll likely piss off the Scots, Welsh and Northern Irish. The Union flag is/was made by combining the England and Scotland (white diagonal cross on blue) flags with a lesser-known one of Northern Ireland (red diagonal cross on white). We apparently forgot the Welsh, but their flag is a dragon and is very different from the others.