Interestingly, there have been no subsequent King Johns of England. Although Portugal had a few kings named John, the name isn’t very popular for kings, even though quite a few popes had that name.
England had an unfortunate young Prince John who was the youngest brother of Queen Elizabeth II’s father and the Duke of Windsor (the guy who resigned). The young man had epileptic fits and was sent to a country estate to get him out of the way, he was an embarrassment to the royals. The King and Queen Mary visited him only rarely. They kept any mention of him out of the news. He died approx. age twelve of an epileptic fit. His only playmates were the children of his caretakers. There’s a documentary about him on You Tube.