The accepted list of Kings and Queens of England contains many anomalies, such as Lady Jane Grey (1553), who is never cited as ‘Queen Jane’ or Edward I (1272–1307), who, if you consider his Saxon forebears, should actually (and more correctly) be cited as Edward IV; but one of the biggest omissions is that of King Louis (1216-7).
In 1216, barons opposed to the rule of King John, offered the English crown to Prince Louis, son of Philippe II of France. Louis, whose claim came through his wife, Blanche of Castile, granddaughter of Henry II, dutifully responded and, on reaching London, was proclaimed king.
By the time of King John’s death, in October 1216, Louis had control of over half the kingdom, but conflict with the supporters of John’s son, Henry, eventually led to a compromise agreement, in September 1217, at which Louis surrendered his claim and returned with his retinue to France (where he was later to rule as Louis VIII).
The main reason cited for his exclusion from the list of English monarchs is because he was not formally crowned, although absence of a coronation should, by right, also exclude Edward V (1483) and Edward VIII (1936).