King Arthur is not fictional, he ruled in the latter 6th Century, but was known by the name Arbuthnot. After the Norman Conquest (1066) many names were 'Normanized', which is why King Henry V is referred to as 'young King Hal' - Henri being the French equivalent of Harold.
King Arbuthnot features in the 'Histories of the Kings of England' first written in the 12th century at Glastonbury Abbey. The original text of this book devotes 3 chapters to Arthur, but the problems arise later. About 40 years later a new edition of this book appears with 2 extra chapters on Arthur, and detail his search for the Holy Grail. It is generally accepted that this was an attempt by the establishment of the time (the Church) to steal Arthur for themselves and declare him as a Christian. Further 'romanticising' of his character was carried out by a French author, Cretien du Toit, which led to the confusion over whether Lancelot was French - he was Welsh.
The fact that the book was written at Glastonbury Abbey possibly also led to the idea that Arthur was from that region, although this again was probably propaganda to elevate the Abbey there to a more important status, in return for the Abbey clothing, housing and feeding the author for over 10 years whilst he compiled the history.
Unfortunately, at the time of Arthur, the Roman Empire had collapsed and many place names are a mix of the Roman and Angle versions. Also, the geography of England has changed substantially over the past 2000 years, with rivers changing, lakes appearing and drying up, and land clearance. With many names being again changed after 1066, it is almost impossible to accurately locate sites from this period.
Arthur did not however fight the Anglo-Saxons, he fought the Danes and the Saxons. The Angles were his predecessors, arriving in the UK before the Romans.