The shortest overland route through France? Why does it imply the tin was taken to Normandy?
Firstly because the Roman source says
Here then the merchants buy the tin from the natives and carry it over to Gaul
The simplest interpretation of which is 'across the Channel'. The voyage to the Garonne is hardly 'over to Gaul'. The Romans hated all the Atlantic trade routes.
The shortest overland route through France is to sail south from Cornwall to the mouth of the Garonne. Sail or barge cargo up the river to Toulouse, the point at which the river turns south toward its source in the Pyrenees. From here cargo is taken overland via the Carcassonne gap through Montpelier to the mouth of the Rhone,
Too efficient surely. The source says
and after travelling overland for about thirty days, they finally bring their loads on horses to the mouth of the Rhone
Ten miles a day on horseback, 300 miles, points to Normandy not the Garonne.
which in ancient times was probably Arles or even Beaucaire but certainly not Marseille. This has been the main trading thoroughfare from the Atlantic to the Med since ancient times.
Precisely so. However the Greeks (aka the Romans) always preferred Marseille. But either works for me.
St Michael's Mount is the best departure point for the Garonne.
Undoubtedly. It is also the best departure point for anywhere in the Mediterranean, including the mouth of the Rhone, by sea, which is why it was so vital for the Phoenician trade. (I haven't dealt with Michael's Mount yet.) The hinterland of Gaul was not part of the Phoenician trade. That's the whole point of the exercise: to show that the Megalithics were an independent trading network. Quite separate from the Phoenicians.
But in general you are making the error of simply considering in isolation the best way to get tin from Western Britain to the mouth of the Rhone. This is certainly by ship all the way. For all I know it is even better by ship and trans-shipment at the Garonne.
But once you start considering that the whole of Gaul, the whole of inland Western Europe is part of the Bronze Age and must therefore have had oodles of tin brought to all parts (whether as tin or as bronze finished goods) then it is obvious that the last thing you would do is laboriously lug everything by sea to the Mediterranean (or to the Garonne) and then start distributing from there.