Well, Elizabeth doesn't quite count. She's Elizabethan, not Medieval. Still, it's not like red hair suddenly popped into existence, ex nihilo, in 1520: There were a bunch, all throughout history. Redheads aren't common, but they're not that rare. read more
Here’s a site that seems a good beginning toward answering your question regarding British red-headed rulers and their spouses, with a book-source for more information: The Myths and History of Red Hair. I don’t know about Continental kings/queens. read more