The number of Keira Knightley square-inch-seconds in King Arthur is probably worth a matinee ticket, if you can tolerate a much muddier than usual Bruckheimer flick.
Its "Arthur" is the (imaginary) lineal descendant of Lucius Artorius Castus (fl. ca. 185 A.D.), himself the only Roman official in Britain known to have borne the name Artorius.
Its "knights" are Sarmatian cataphracti, an interesting (if ridiculously presented) take on a theory proposed first (AFAIK) in the 1920s, and elaborated well beyond all sanity in the terrifically interesting From Scythia to Camelot by Littleton and Malcor.
Its "Britain" is the Land of Silly Stupid Pretend.
Should anyone actually be interested, heaven forfend, in actual Arthurian scholarship, I heartily recommend
Tom Green's Arthurian Resources and, for the uses the Arthur stories have been put to -- including by Nennius and the composers of the Welsh tales -- N.J. Higham's King Arthur: Myth-Making and History. Higham also does a relatively good job of summarizing the state of scholarly knowledge about Arthur.
Do not come to either Green or Higham with a romantic spirit, however, or they shall crush you like Arthur may or may not have crushed the Saxons, possibly at Badon Hill, wherever that was, around 540 A.D. or thereabouts, if he existed at all, which there's barely any scholarly reason to say that he did.
But, to paraphrase President Harding, I love King Arthur whether he rode or not, especially if he's got Keira Knightley painted blue in the frame.