Wales 38
Image by GLADMAN

moss ;"I thought of this legend,but couldn't decide if what I remembered was from The Crystal Cave by Margaret Stewart, and so part fiction. What about the legend of Bran the Blessed, his head is buried at White Mount and if it ever leaves off its placement there, guarding our shores, Britain will fall. (Bran means raven - which gave rise to the obfuscation that Britain would fall if the ravens left the Tower of London at White Mount). That aside, the head and the stone are closely connected in celtic myth and legend."

As Gladman says Welsh history is extrordinary complicated without bringing in Nennius, Geoffrey of Monmouth and Gildas the monk into the discussion;)
As you say Cunedda migrated down from Scotland and came after Vortigen as a leader, interestingly if you read the Vortigen study link on the Dinas Emrys page, the sacrifical boy/magician/next leader who told the tale of the two dragons - Ambrosius, may be part of the legend of Stonehenge as well, a meeting having take place at Amesbury.
Whatever this 'dark age' when all was up for grabs as far as storytelling is concerned had I believe the one mention of Arthur up in Scotland fighting a battle, now whether he came down with Cunedda might explain his presence, I like to think Arthur was a king at Caerwent, but Tintagel is another stronghold for him.
Trying to remember the story of the warriors/king who slept in a cave, and if you woke them up, he would become the next king of Wales? or have I got it wrong..

Earliest mention of Arthur is in the Goddodin, very brief. Arthur legends themselves only had to travel with Cunnedda for them to migrate to Wales. There are certainly plenty of books now which make a good case for Arthur being a hero from the North, placing most of the action up here. And it's true that at the time of Monmouth (look at who his sponsor was) the Scots were hated, and no one would place a great story as happening here for fear of offending the English King.

Personally, I think the story goes back much further, and was retold over and over, with changes as new heroes were placed in the roles of the old heroes. All hero warrior kings become one. It's the nature of the story. The archetypes keep being reborn. I can see echoes of Jack the Canny Lad of many a celtic traveller tale in Captain Jack Harkness. In TV I see the characters from my stories - the unlikely warrior who is the "last best hope" and the time traveller "just trying to get home".

Your point about the king under the hill is a classic example. All over britain there are hills which claim the king (sometimes arthur) is sleeping under it with his warriors, and they will awake when they are most needed. Some have changed to say they will awake on judgement day now. Some have even changed in Scotland to say the sleeping Warrior/King is William Wallace or Robert the Bruce. Stories became localised, as part of the storytellers craft. You stand before an audience telling the story. Eye to eye, voice to ear, imagination to heart. Around a campfire you declare "there, under THAT very hill, is where he is said to lie!!".