I had never heard of a French prince becoming an uncrowned King of England until I came across this –
The future Louis VIII of France briefly won two-thirds of England over to his side from May 1216 to September 1217 at the conclusion of the First Barons’ War against King John. The then-Prince Louis landed on the Isle of Thanet, off the north Kent coast, on 21 May 1216, and marched more or less unopposed to London, where the streets were lined with cheering crowds. At a grand ceremony in St. Paul’s Cathedral, on 2 June 1216, in the presence of numerous English clergy and nobles, the Mayor of London and Alexander II of Scotland, Prince Louis was proclaimed King Louis of England (though not crowned). In less than a month, “King Louis” controlled more than half of the country and enjoyed the support of two-thirds of the barons. However, he suffered military defeat at the hands of the English fleet. By signing the Treaty of Lambeth in September 1217, Louis gained 10,000 marks and agreed he had never been the legitimate king of England.[44] “King Louis” remains one of the least known kings to have ruled over a substantial part of England.[45].
The fact he was not crowned seems immaterial Edward V and VIII and Jane Grey were also not crowned. Paying him off with 10000 marks (approx £666.666 pounds) must have hit the exchequer at the time? Could the loss of the gold by King John in the Fens be the early work of a ‘spin doctor?”.