Egfrith

Egfrith was — according to a priest who once performed a British coronation — the King of Mercia. In 785, he was the first king of a part of Great Britain to be anointed with holy oil during his coronation ceremony. (PROSE: The Dying Days)

Behind the scenes
Egfrith is another example of how the DWU does not always follow real world history precisely. It is true that Egfirth was the first King on British soil to be anointed during his coronation. But that coronation happened in 787, not 785.