The Kennet Long Barrow is similar age, shape and construction:
https://en.wikipedia.org/wiki/West_Kennet_Long_Barrow
Similar barrows exist across southern England, and in the wider Atlantic coast region, from Spain to Brittany and Ireland:
https://en.wikipedia.org/wiki/Cotswold-Severn_Group
The only unusual about King Arthur's Hall is the apparent absence of burial chambers.