It's a gorgeous, gorgeous book - you must read it.
It's all about how they started to interwine the myth of Arthur and Avalon etc with place - Glastonbury just one of the places.
But let me show you this photo - it's an artist photo so I'll just give you the link
https://colin-rayner.pixels.com/featured/glastonbury-tor-above-the-fog-colin-rayner.html
No wonder people imagine the tor as avalon - the somerset levels are pretty flat and marshy and very wet in the winter. It's easy to imagine it as Avalon.
But then, the church were just doing their propaganda thang to bring in the gold and prove how amazing the place was. Joseph of Arimathea apparently threw his staff in the ground and lo - a thorn tree grew. Of course it's bollocks. But hey, it got some funding for the monastry and the patronage of pilgrims flocking to see it.
What a crazy place with all the intrigue of an Arthurian legend.
That is how I would imagine it, and I could see how story tellers would be inspired by it, especially in the olden days before mass media.