I have no idea why that would be believed impossible. The Phoenicians are reported to have circumvented Africa and when the voyaging party returned they report

I have no idea why that would be believed impossible. The Phoenicians are reported to have circumvented Africa and when the voyaging party returned they reported Phoenician colonies on the Atlantic coast of Africa before entering the Mediterranean. This was done under contract for an Egyptian pharaoh, and from what I understand, the claim that the sun was on their right side as they rounded the southern tip of Africa, although viewed with derision in the ancient world is now widely accepted as evidence they did indeed make this trip. At any rate, they were amazing sailors for the ancient world and it would be much harder to believe they didn't make the shortish trip up the coast of Europe to the British Isles then the reverse in my opinion. But as for tin, I thought it was well known tin came from the British Isles. Could this tin have been brought in earlier when Bronze Age commerce was at it's height? Did the Bronze Age collapse really collapse _all_ trade and commerce and travel? Doubtful. Anyway, interesting find but not really a novel story.