Two of the sites that stand out in King Arthur stories are Glastonbury and Cadbury Castle. Whether King Arther realy lived and died at either of these two places has yet to be proven, but there is a lot of circumstantial evidence.
Cadbury Castle Cadbury Castle is near the town of South Cadbury in Somerset. Cadbury Castle is a good example of an Arthurian-age hillfort. Archeology shows "Tintagel" pottery here, and it was re-fortified at the right time.
Glastonbury Glastonbury is a town in Somerset. It is set in a cluster of hills and was at one time almost an island as the surrounding Somerset Levels were wetlands. In Celtic times was a centre of overseas trade. Glastonbury Tor, the highest of these hills had a place in myth and ritual as the door to Annwfa, the Otherworld. And this has probably led it to being identified in the King Arthur legend as The Isle of Avalon.
Marlborough Merlin's Mount is in the grounds of of Marlborough College. "Marlborough" could possibly (but is unlikely to) be derived from the name "Merlin"; Marlborough latinized is Merleburgia.
Pomparles Bridge Pomparles Bridge crosses the River Brue just south of the town of Glastonbury. The derivation of the name is from the French pont perilleux, "perilous bridge." Hence the connection to King Arthur.
Portchester Castle Portchester Castle is in Hampshire and is on Portsmouth Harbor. Its King Arthur connection is somewhat tenuous and rests on a Welsh poem from the Black Book of Carmarthen
Stonehenge Stonehenge is in Wiltshire. Stonehenge is thought to date from around 3300 BC, but Geoffrey of Monmouth has it being constructed by Merlin.
Wansdyke Wansdyke is a large bank and ditch earthwork that extends (in fragments) from just south of Bath to the Savernake Forest south of Marlborough. The dyke is generally accepted as post-Roman and pre-Saxon, which fits the King Arthur time frame.
Winchester Winchester is in Hampshire, and this version of the King Arthur Round Table is in the Winchester Great Hall. The table is thought to have been constructed during the reign of Edward I. Both Mallory and Lelend mention the Winchester table
The strongest King Arthur connections in Southern England are in the Glastonbury area.