King Arthur is a legendary ruler from Saxon England, meaning he ruled before the Norman invasion of 1066 and after the Romans withdrew from the territory. To be legendary means to be part fictionalized or mythical and part real and historical.
The stories of King Arthur are believed to be based on a real person, but they are largely fictionalized and serve the function of nation building: Arthur, in many ways, represents the highest ideals of British society.
These fall into two categories: first, Arthur as superior warrior who could protect the country from invaders of all sorts. In this sense, he functions as a Beowulf figure, protecting civilization from the forces of darkness, be they dragons or foreign armies.
Second, however, Arthur is connected with nation building and establishing justice. At least in story, he pulls together disparate groups into a unified whole willing to follow him rather than fight amongst themselves. This is symbolized by the image we most fully and immediately associate with Arthur: his Round Table, a mythic object that was being reproduced in England as early as the 1400s. It became a symbol of his greatest legacy, monarchy joined to notions of unity and equality as represented by everyone having a seat at the table designed to level hierarchy. It is hard to reconcile one man rule with everyone having a voice, but this is an ideal the British have long struggled to achieve.