The best recent Cornish historian is Philip Payton; see for example, his Cornwall: A History (1996/2004) in which he writes "Tales of Phoenicians and Carthaginians making their way from the Mediterranean to trade for Cornish tin, like claims of Mycenaean contacts in the Bronze Age, are routinely repeated but not attested in hard fact." (47)

Regarding Romans and Cornwall, Payton indicates that there is some evidence of Romans trading with Cornwall for tin before invading Britain. The inspiration for invasion was hardly access to tin (which remained useful in the Iron Age because brass and bronze were still useful alloys, as they remain today). Instead, I think we can look at the Roman policy "let's conquer the next village to suppress sources of dissent that is diffusing into our empire." Britain was simply too close, too rich, and too tempting in many ways not to invade. Britain - especially the southern part - has always been an agricultural paradise that tempted the Romans. But like the Anglo Saxons, the Romans must have realized that Cornwall, the place "beyond the Tamar," was remote and if left to its own and treated with a certain amount of deference would provide tin as a trading partner - without resorting the violence of a logistically-difficult occupation.