Excalibur
Geoffrey of Monmouth, in his "Historia Regum Britanniae" ("The History of the Kings of Britain", c. 1136), Latinised the name of Arthur's sword as "Caliburnus" (potentially influenced by the Medieval Latin spelling "calibs" of Classical Latin "chalybs", from Greek "chályps" ["χάλυψ"] "steel"). Most Celticists consider Geoffrey's "Caliburnus" to be derivative of a lost Old Welsh text in which "bwlch" (Old Welsh "bulc[h]") had not yet been lenited to "fwlch" (Middle Welsh "vwlch" or "uwlch"). In the late 15th/early 16th-century Middle Cornish play "Beunans Ke", Arthur's sword is called "Calesvol", which is etymologically an exact Middle Cornish cognate of the Welsh "Caledfwlch". It is unclear if the name was borrowed from the Welsh (if so, it must have been an early loan, for phonological reasons), or represents an early, pan-Brittonic traditional name for Arthur's sword.
Arthur Uther Pendragon
After reading a book on King Arthur by the occultist Gareth Knight, Rothwell saw similarities between himself and the legendary king and came to believe that he was King Arthur reincarnated. On 11 June 1986, he officially changed his name to Arthur Uther Pendragon by deed poll. Shortly afterward, he bought a sword called Excalibur in a Farnborough shop; its seller stated that it had been the prop in the 1981 film "Excalibur". He designed a white robe with a red dragon on it and began regularly wearing it.