The tales that have been handed down, and eventually put into words, on the subject of King Arthur, tend to be, to say the least, somewhat vague.
If King Arthur is all myth and no historical fact, then its academic as to where the places actually were in Britain. However if there is some historical fact behinmd it, then where did the adventures of himself and his Knights of the Round Table actually take place.
Wales and Cornwall appear to have the strongest historical claims, thouh other sites throughout Britain claim King Arthur memoribilia ( Southern England, Central England, Northern England, Scotland)
The links below are to some of the better known sites in Arthurian Legends, which in turn will lead you to places that might be their origin..
Camlann, last battle of King Arthur
King Arthur and the Round Table
The biggest concentrations of places with King Arthur connections appear to be in Cornwall and Wales.
The Legend of King Arthur