Actually, the Kingdom of Asturias far preceded both the Kingdom of Leon and the Kingdom of Castile.
The Kingdom of Asturias was established in 718 AD and had it's capital in Oviedo.
As the conquest moved south, the city of Leon, originally established by the Roman VII Legion (Legio Septima Gemina) was conquered in 742 AD. Over time, the old Roman name of "Legio Septima" had been corrupted to "Leon". Thus, the old Roman name for "Legion" morphed into "Lion".
As time passed, Alfonso III, King of León, divided his Kingdom into Asturias, Galicia and Leon between three sons, Ordoño, Fruela and Garcia upon his death in 910 AD. Thus began the "Kingdom of Leon" as a separate kingdom when it was inherited by García I of León. In the end, through the deathof Garcia and politics, Ordoño ended up ruling the united Kingdom of Leon, Galica and Asturias with the capital now in Leon.
Castile was originally newly reconquered territory on the eastern frontier of Leon and was just a county of Leon ruled by Counts of Castile that were, legally, vassals of the King of Leon.
With the passage of time, the Counts of Castile became more and more powerful until they were, in effect, independent although they remained, legally, vassals of the Kingdom of Leon. The rulers of Castile remained as "Counts of Castile" until Ferdinand I of León and Castile, Count of Castile, through civil war and marriage, gained control of both Castile and Leon and established the Kingdom of Castile in 1035 AD.
Sancho I of León (the Fat), was the son of King Ramiro II of León who assumed the throne of Leon in 956 AD and died in 966 AD. His reign was rather inglorious, being deposed once by the nobility led by the restless Count of Castile Fernán González.
The tale of knights from Cornwall might make a nice English legend but it is, in the end, just a legend with no basis in historical reality.
You are leaping ahead a couple of centuries after King San Cho I
But, I digress, we can go back to 700 BC or thereabouts when the Lords of Galicia departed the Dead Coast and invaded the British Isles taking with them their Basque and Celt-Iberian slaves.
They founded the lines of kings in all of Britain down to the days of Boadica. After the departure of the Romans the Britons raised back up their own kingdoms ~ Alba, Brittain, Cornwall, Wales, Scotia ~
They returned to Spain just on the heels of the Moslem advance into the Iberian peninsula.
So, no, there's no "English" fairy tale. On the other hand, there is Galician history, and their own sources clearly denote the Cornish invasion which began the Reconquista.
Later history written in the rough tongue of Roman slaves is the part that's questionable.