In my sprawling tree which is nearing 2500 people, there is an interesting distant ancestor to Robert E. Lee of Virginia (I haven't placed him in my tree, however). Anyway, Isabel de Cornwall, the ancestor I referred to, has her father's line going straight back to King John via the younger brother of Henry III. And her mother's maternal line goes back to (I forget her name off hand) an illegitimate daughter of King John and one of his mistresses, who married Prince Llewelyn-the-Great of Wales.
Yes. And anyone who might be descended from King John's illegitimate daughter Joan and Llewyelyn The Great should remember that Llewyelyn's side of the family was higher and older than the Plantagenet side of the family.