The historical King Arthur would have been a Romano-Britain. They were Roman immigrants who had intermarried with local Celts over the centuries.
He would have fought invading Angles, Saxons, and Norsemen.
So the historical King Arthur would not have been English at all. Today's Irish would have a greater claim to him than the English. Think of Arthur as an early Irish nationalist. (Or Italo-Irish nationalist?)
Merlin and Morgana are Celtic names, and thus these fictional characters are part of the older stories.
Lancelot and Guinevere are French names. These characters were probably imported into Britain after the Norman conquest and grafted into the Arthurian tales.
Guinevere is an adaptation of the Welsh name, “white shadow” — later, the name of a Peter Gabriel song.