There are many sources of tin, but mining tin becomes more difficult over time, especially in areas of low abundance. It became cheaper to transport the tin from areas of high abundance.

Traders along the Mediterranean would start paying more for tin (in barter for pelts, amber, salt et cetera..) A trader along the Rhône would know tin can be obtained in Massif Central and buy from villages that occasionally mine it, motivating them to mine more. However after decades with the easy to access tin extracted the villagers would not bother until prices rose again.

Soon traders in Brittany noted the price of tin was higher to the south, enough to begin routinely moving it to the mouth of the Garonne river where it could be taken east to the Mediterranean. In turn traders in Cornwall heard prices were higher across the channel. Kind of like pass the parcel, tin made the 2000 mile way to the fertile crescent through multiple hands.

A tin miner in Cornwall need not know where Egypt is or even its use in bronze. No one needed to travel the whole route, Greek explorer Pytheas of Massilia (Marseilles, near the Rhône) followed the source of tin to Cornwall, but this was apparently a rarity as he was as far as I know the only written source until the Romans linked it all under one polity and it became trivial.