I read recently that Edward I was named Edward by his father, King Henry III, after the Anglo-Saxon king, Edward the Confessor, and that the selection of an Anglo-Saxon name was highly unusual for Anglo-Norman aristocrats. This of course makes sense given that the Normans had their own culture and customs separate from those of the Anglo-Saxons. This would have been questionable were it not for the so-called Cult of Edward the Confessor in that era, who had become venerated for his piety in the generations after his death. With this, I was thinking that another early King of the Anglo-Saxons was Edward, son of Alfred the Great, the latter of whom is often recognized as being the first King of England, even if just essentially and not literally by title.
From this, it had been important for the Anglo-Normans to secure and maintain the idea that William the Conqueror’s claim was unquestionably legitimate, because if it wasn’t, then of course the reign of the Normans in England itself could be found to be completely illegitimate. Thus the Normans and the Anglo-Normans thereafter maintained the idea that William had in fact been the selected successor of Edward the Confessor, whose title was King of England. So why was the later King Edward ‘the Longshanks’ I styled as ‘the first’ if the Anglo-Normans recognized that kings like Edward the Confessor and those before him (namely Edward, son of Alfred) were kings of England too? How was any of this argued, rationalized, or justified?
Some good answers on this subject here from u/Valkine and here from u/Miles_Sine_Castrum
The short answer is that Edward II was contemporaneously known roughly as Edward, son of Edward, but this became cumbersome for his son, Edward. Thus, we get the numbering. Also, as a note, Edward I, Longshanks, would actually be Edward IV! after Edward the Elder, Edward the Martyr, and Edward the Confessor.