True, there isn't any written, by them, evidence for Druidic practices; however, there are ancient Celitic stories, that though they have changed through the ages ( a la the game "TELEPHONE" ), regarding certain things.
Through ancient Roman writings, we DO know that the holy and oak trees were "sacred" to the Druids, AND even to this day, some aspects of Christmas and Halloween are STILL in practice today.
And then there's the newly found, OLDEST tales re Merlin, that are far different from the once oldest English and French versions.
Don’t forget the bog bodies.
Boorman’s gorgeous Excalibur included the mythological/sacred kingship aspect wonderfully.
The land fell barren because Arthur was “ broken” and needed to be healed by the Grail.
Even the movie The Fisher King ( love it) touched upon cleverly masked Arthurian mythology
I could even drag in the drawing and quartering and distributing his bits around the countryside of The Wallace as a backfiring bad idea by Longshanks because the heathen peasantry understood the sacred kingship aspects and were inflamed to fight.