Although early British monarchs may have adopted second names, these were not hereditary and were more in the way of nicknames.
The Irish and Welsh rulers kept this system and did not adopt surnames. The first English monarch to use a hereditary surname was Edward IV. He used the surname Plantagenet to emphasise that he was descended from the elder branch of the royal family – unlike Henry VI who came from a younger branch.
The name Plantagenet refers back to Henry II who came to the throne in 1154. Although it had not been used in formal documents before Edward IV, it must have been in informal use during that period as it was clearly of practical benefit to Edward.
The first Scottish king to have a surname is generally said to have been John de Balliol, who gained the throne in 1292 after the death of Margaret, heiress to Alexander III. However, John’s surname was more of a title indicating his hereditary lordship, an objection that could also be made to Robert the Bruce (ruled 1306–1329). The first Scots king undeniably to have a surname was Robert Stewart (or Stuart) who gained the throne in 1371 and ruled for 19 years. His surname derived from the family’s traditional role as Stewards of Scotland.
This Q&A was answered by historian and author Rupert Matthews, in June 2016.