u/sunagainstgold and I talked about the second half of your question here (Sun's answer being part of last April Fool's "AskHistoricalFiction"), and I'd be happy to answer any follow-up. As for the first part, no data exists to give you an exact number, but there haven't been any large population shifts in England to cause a major change in the prevalence of red hair between then and now, as far as I know. It's possible that there's been some level of genetic drift or change, but that's mostly a scientific question, not a historical one.