There is not much in the written sources about tin exporting in pre-historic Britain. A single reference is more or less all there is:
The inhabitants of that part of Britain which is called Belerion ... prepare the tin, working very carefully the earth in which it is produced. The ground is rocky but it contains earthy veins, the produce of which is ground down, smelted and purified. They beat the metal into masses shaped like knuckle-bones and carry it off to a certain island off Britain called Ictis.
During the ebb of the tide the intervening space is left dry and they carry over to the island the tin in abundance in their wagons ... Here then the merchants buy the tin from the natives and carry it over to Gaul, and after travelling overland for about thirty days, they finally bring their loads on horses to the mouth of the Rhone.
[Diodorus Siculus writing around 1 AD.
Belerion is identified elsewhere by Diodorus as the western angle of triangular Britain (Cantium, ie Kent, being the eastern angle). Diodorus is clearly saying that the tin ingots are being produced where the tin ore (1) is being extracted and since transporting tin ore, as opposed to tin ingots, is prohibitively expensive in any age, this can be taken as a given.
Although Cornwall is exclusively associated with tin mining today, the ancient industry was rather more centred in neighbouring West Devon, especially around Dartmoor. In fact Dartmoor and the rest of the moors of the English West Country are the result of tin mining – or rather the deforestation of the countryside to provide fuel for the tin-smelting industry.
No doubt deforestation took place everywhere but only in areas of higher altitude, and therefore of greatest soil erosion, were the results permanent. Dartmoor, Exmoor, Bodmin Moor and the rest show, rather better than archaeology, where ancient tin mining (and smelting) took place. Belerion then can be considered as the entire western peninsula of Cornwall and West Devon and essentially co-extensive with the tin industry.
(1) Tin ore is just ‘rock’, the stuff miners dig out and bring to the surface. The tin, characteristically in flecks and veins and only making up at best a few per cent of the whole, can then be extracted by processing, ‘smelting’, the rock. Tin ingots, more or less pure tin, are the end product of the processing.