When he is a hereditary head of state under a different title, of course. There are such people around the world today but Britain had them for a few years.
The first was Oliver Cromwell, the great-great-great-nephew of Thomas Cromwell. As he was finalising the execution of Charles I in 1649, he announced that “the office of King is hereby abolished”. Four years later, he accepted the title of Lord Protector and Defender of the Realm, previously only held for three under age Kings by their closest adult male relatives, of whom Richard of Gloucester was one. When Oliver Cromwell died in 1658, he was succeeded by his son Richard, whom he had evidently nominated in advance.
This article reminds us that the three kings named Richard all died of violence or intentional neglect at an early age. Richard Cromwell, although he was only a de facto monarch for about nine months before resigning (abdicating?) but lived on until 1712 when he was eighty-five, spending all but twenty years of his retirement in his own former realm, but his royal connections may not end there. His mother was Elizabeth Bourchier and is likely to be connected to the original noble family by that name, into which Richard’s aunt had married .