The Roman Map of Britain Antivectaeum promontorium The promontory opposite Vectis (or opposite the Vectae) - Land's End, Penwith, Cornwall

Antiouestaion akron to kal Bolerion Antivestaeum sive Bolerium promontorium vars. Antiouesteon Antivesteum and Bolairion Bolaerium (Ptolemy II 3 2) [for Antiouektaiwn]
Belerion Belerium (Diodorus Siculus History V 21)

    The review of Antivestaeum in PNRB pp 252-3 considers various interpretations of vesta as hearth, a beacon, or something dedicated to the goddess Vesta. Anti- is Graeco-Latin 'opposite', and its use in place-names to denote the physical relationship between a mainland opposite an island is well documented. Our perception of Ptolemy's map directs us to consider Land's End as the recorded promontory and recognize that it was opposite the Isles of Scilly. The Scillies were thus the site of the vesta.

    But there is the possibility that Antivestae- is a corrupt Antivectae-, that is, 'opposite Vectis'. If so, the corruption was quite early and would have been by way of a kappa  K read as a lunate sigma C, the form used at the time of Ptolemy; or a Latin C inadvertently transcribed as a lunate sigma C. Among the six islands Pliny the Elder places inter Hiberniam ac Britanniam 'between Ireland and Britannia' IV,103 is a Vectis. It does not sound like he is writing about the Isle of Wight Vectis, but another. Further on, Pliny writes, recounting Timaeus, of insulam Mictim (the island of Mictis) in which tin is produced. Diodorus Siculus, following the work of Timaeus, writes of an Iktin Ictim (acc.):

    'The inhabitants of Prettanike who dwell near the promontory called Belerion are especially friendly to strangers and, because of their intercourse with foreign merchants, civilised in their dealings. These people prepare the tin, treating the earth which bears it skillfully. This while rocky, has earthy layers in which they work the ore and purify it by smelting. Then they hammer it into the form of astragali and convey it to an island lying near Prettanike called Iktis; for the area between being dry at ebb tide, they convey the tin in large quantities to it in wagons. And a peculiar thing happens concerning the neighbouring islands lying between Europe and Prettanike: for at flood tide the passage between them is full and they appear as islands, but at ebb tide, when the sea flows back and leaves a large dry area, they are seen as peninsulas. From there the merchants buy the tin from the natives and carry it across to Gaul.'

This Ictim has been reasonably  identified by some as St. Michael's Mount (SW5130) Cornwall. PNRB p.488 considers the possibility that Ictim is properly Vectis (acc. *Vectim or Vectem). Förster cited Nennius on the Isle of Wight 'quam Britones insulam Gueid vel Guith, quod latine divorcium dici potest'. Latin divortium is 'a separation, a place where a road divides, a watershed'.

Consider that rather than 'a separation', a Vectis may be 'the part separated' (from the mainland), and might simply be an early generic term for an off-shore island.

    Having written the above, I was reading Edmund McClure's 'British Place-Names in their Historical Setting' (1910). He was recounting the Anglo-Saxon Chronicles when the content of two lines struck me.

    "The year 997 is marked by the arrival of the enemy at the mouth of the Severn (Sćfern Muthan), whence they harried Devon, Wales, and Cornwall. Then, after again ravaging Watchet, they went round the Land's End (Penwiht Steort)2 to the mouth of the Tamar (Tamer Muthan) and proceeded as far as Hlidaford,3 burning and slaying wherever they went, Ordulfes Minster at Tavistock (Tefinge stoc)4 being given to the flames by them.
    The next year (998), the enemy proceeded eastward to the mouth of the Frome (From-muthan), whence they raided Dorset on all sides. They proceeded next to the Isle of Wight (Wiht-land), Hampshire, and Sussex,.....

    2 Penwihtsteort. This appears in Simeon of Durham as Penwithsteort. Steort is an English word meaning "tail", as in the bird Redstart. It is applied to a prominent headland, as "Start Point". Penwith, the Cornish name of the district, came to be applied to the Land's End. Tol Pedn Penwith [Gwennap Head] is another point near. The meaning of with or guith is not clear. It could hardly be Cornish Gwedh, Welsh Gwydd, meaning "woods", for no woods could well grow there. There is, however, no nearer term, and the Welsh Penwedic seems to be of similar import = wooded point. The name of Land's End in the Welsh Triads is Penbryn Penwaeth, and also Penwedic yng Ngherniw (Myrv. Arch.)."

    The answer to the meaning of wiht/with was right there all the time, the with or guith is the same as that of Nennius 'quam Britones insulam Gueid vel Guith'.

    What remains unanswered is the identity of Vectis/Vectae - the Scillies or the islands of Mount's Bay?

...But as touching this name Antivestaeum, I was wont now and then to doubt whether it savoured not of some Greek originall. For seeing it was a common and usuall thing with the Greeks to impose names upon places taken from the names of such as were opposite unto them, not only in Greece it selfe, where they have Rhium and Antirrhium, but also in the Arabian gulfe, where there is Bacchium and Antibacchium, as also upon the gulf of Venice Antibarrium, because it looketh towards Barrium lying over against it in Italie, I searched diligently whether any place named Vestaeum lay opposite unto this our Antivestaeum... William Camden Britannia (1607) with an English translation by Philemon Holland