From what I read.. in ancient ethnic groups specifically European ethnic groups.. red hair was some sort of signal of having a wild temperment, filled with anger and violence. The women are supposed to be basically... (no offense anyone) promiscuious home wrecking types. Basically, slutty.
It's supposed have come about because of the fact that many Celts had red hair and they were known for their violence and hostile temperments. Given the Scots descend from them and many have red hair as well as many Irish.. it just spread in Europe these stereotypes about red hair.
And people with red hair get labeled in a negative way in those countries where those people originated. Hence the UK issues.
I think we've all heard the phrase, "Fiesty red-head" before. It comes from those sorts of sagas, legends, and myths.