The King of Northumbria was the title given to the ruler of the Saxon kingdom of Northumbria.
In the late 9th century, Alfred the Great, the King of Wessex, was engaged in a war with King Guthrum. By the time of Alfred's meeting with the Sixth Doctor, Guthrum had already successfully invaded Mercia and Northumbria to become their kings. Alfred told the Doctor that, after the disasters at Anglia and Reading, there were those who told him to give up and make peace with the Danes. The Doctor assured him that he was not fighting in vain and that Guthrum would become a Christian, ending hostilities.
The Doctor also prophesied to Alfred that, although his kingdom of Wessex was currently alone, it was to be joined by many other territories which would coalesce and reform into a greater whole with Alfred becoming king of not just of a region but a whole country, one which was eventually to be called England. (PROSE: The Man Who Wouldn't Give Up)