Charles III

Charles III was the king of the United Kingdom who succeeded Elizabeth II. Charles reigned with his Queen, Camilla, and was succeeded by William V. When the Tenth Doctor found that Edward VII had disappeared along with Balmoral Castle in 1902, he warned that the future of the royal family, along with the rest of history, was threatened. Saying that, along with the rest of his successors, there would be no Charles III. (PROSE: Revenge of the Judoon)

At some point in the late 20th or early 21st century, a king was on the throne of Great Britain, presumably Charles. (TV: Battlefield) However, this would appear to have been only temporary (and for reasons unknown), as Elizabeth II was again the monarch by the end of the first decade of the 21st century. (TV: Voyage of the Damned, Turn Left, Planet of the Dead)

In 2020, the British Army had a group called the King's New Delta Squadron. (AUDIO: Black and White)

Behind the scenes

 * When a heir accedes to the British throne, he or she may choose the name they will rule under, and it does not necessarily have to be their birth name. There is no guarantee that when Charles takes the throne in real life that he will, in fact, choose to be Charles III.
 * A recurring way of indicating it's the near future is to have a King (Charles) instead of Queen Elizabeth: this was the idea behind Battlefield and "the King" is briefly mentioned in AUDIO: The Longest Night.