Perhaps a variant of the Roman spatha. The spathas resembled a lengthened gladius, and increasingly supplanted the latter as more and more Franks, Saxons, Goths, etc were recruited in the Western portion of the Empire as auxiliaries and cavalry grew in prominence. In an account of the botched 1st century rebellion of a British chief named Caratacus, Tactius describes the blades carried by Roman forces as "spathis" (sec. 35).
With early use attested to amongst Roman and auxiliary forces in Britain, spatha-esque weapons would certainly have been in use by the local populace (as well as the Germanic tribes that Arthur is said to have fought) by the time of Roman withdrawal in the early 5th century.
The attached images courtesy of a historic sword reproduction company and a reenactor, provide an idea of what such weapons may have looked like.
