By far the most probable derivation of this word is that given by Bochart, from the Phoenician Baratanic (country of tin), contracted into B'ratan'. The Greek Cassiterides (tin islands) is a translation of Baratanic, once applied to the whole known group, but now restricted to the Scilly Isles. Aristotle, who lived some 350 years before the Christian era, calls the island Britannic, which is so close to B'ratanic that the suggestion of Bochart can scarcely admit of a doubt. (De Mundo, sec. 3.)
Pliny says, “Opposite to Celtiberia are a number of islands which the Greeks called `Cassiterides' ” (evidently he means the British group). Strabo says the Cassiterides are situated about the same latitude as Britain.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894