From "The English and Their History" by Michael Tombs:
It has been suggested, a little teasingly, that the inventor of the English was Pope Gregory the Great, in around 580--though he might have got the idea from the historian Procopius (circa 500-565), who described one of the peoples of "Britta" as the "Angiloi." Gregory, noticing fair-haired slaves for sale, was told they were Angles: "Not Angles, but Angels" was his famous pun. Whether or not the story is true, Gregory not only sent a mission of forty monks under the prior of a Roman monastery, Augustine, in 596 to convert these angels with dirty faces from their Germanic heathenism, but he firmly decided that they constituted a single people, and only ever referred to them as "Angli." The organization of the Church that followed Augustine's mission was based unwaveringly on that assumption. There came to be a single Church of the gens Anglorum, with two provinces in Canterbury and York. This Church transcended the boundaries of the various kingdoms...No less important, the Church did not include all the islands, but excluded Ireland and nearly all the turbulent lands that would eventually become Scotland and Wales.
This story was given historical shape by Baede, or Bede, a monk of the great Northumbrian monastery of Jarrow, who was "indisputably the greatest historian of the Middle English Ages, and arguably the greatest historian of all time." His Ecclesiastical History of the English People, written in Latin and finished in 731, gave an intellectual and religious significance to this still hypothetical nation...