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.
The Excalibur Alternative
As the massive Federation battleships are about to destroy the comparably tiny human ships, even bigger spaceships appear and destroy the Federation ships easily. The rescuers of Earth are the Avalon Empire, the civilization founded by the English who have grown in numbers and, using the technology found on the captured Guild ship, have advanced far beyond the Federation. For centuries, they have prepared and lay in hiding so could return at the moment of Earth's greatest need (just like the sword Excalibur in the legend of King Arthur, which gives rise to the title of the book).