How common is red hair in England?

It is quite common in England too. I am guessing here but I would suggest it is probably not all that far behind the 'celtic' countries, maybe around 6% or 7% (Ireland for instance is 10%.) It is probably higher than most if not all the mainland European countries with the possible exception of the Netherlands, which also has a lot of red. As you say it is probably in most cases an indigenous trait that has been there many thousands of years, especially since it is now well known that most English people descend from exactly the same prehistoric stock as their 'celtic' neighbours rather than Anglo-saxons etc.