Do archaeologists or other relevant experts have any sense of how common these things were / what fraction have been destroyed either through natural processes or human activity? Are there a smallish number of these sites because they were always rare (and took a lot of labor to build) or were there a lot more of these but like a lot of them were deconstructed by iron age farmers or something?
Australian aboroginal artifacts date back 10000s of years but not sure if they have something like these rock structures from that long ago. But I wouldn't be suprised.
I meant unrelated to "Kennet". We seemed to be talking about West Kennet long barrow by this point.
I'm not sure which two of the three things (Kinnettles, Mildenhall, and Cornish names) you're comparing. If you're saying it's outrageous to find a Celtic place name in Suffolk, you're right, except this is a name from Roman times, when Romans were Romanizing Brittonic Britons and the Anglo-Saxons hadn't yet got a look in. And river names tend to be the oldest names and full of debated and unexplained elements, such as the "Or" in Orwell, and the Ouse.
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.