The British royal family isn’t just famous for its castles and crowns; it’s also a serious business powerhouse. According to a recent report by The Sunday Times and Channel 4’s Dispatches program, King Charles III and his son, Prince William, aren’t just relying on the “sovereign grant” from UK taxpayers. They have extra sources of income that bring in millions every year—thanks to two ancient estates, known as the Duchy of Lancaster and the Duchy of Cornwall.

How King Charles and Prince...