Joseph of Arimathea
As far as can be ascertained the Abbey was founded in the 7th century by early Christian Britons, but according to legend it was actually founded by no less a personage than Joseph of Arimathea - the man who donated his prepared tomb for Jesus after the crucifixion. Joseph's connection with Britain in general dates to allusions made in various Roman histories
Rabanus Maurus - a Roman historian describes how Joseph (among other disciples) was sent forth by Jesus and made his way to Britain:
"Leaving the shores of Asia and favoured by an east wind, they went round about, down the Tyrrhenian Sea, between Europe and Africa, leaving the city of Rome and all the land to the right. Then happily turning their course to the right, they came near to the city of Marseilles, in the Viennoise province of the Gauls, where the river Rhône is received by the sea. There, having called upon God, the great King of all the world, they parted; each company going to the province where the Holy Spirit directed them; presently preaching everywhere"
This description might not include anything to do with the A39 (the route favoured by modern visitors) but actually matches more or less the classic description of the Phoenician trade route to Britain, despite seemingly being concerned more with France.
Later historians and mythmakers would embellish these stories with greater specificity, placing Joseph at the foundation of the Abbey and at the centre of a wealth of tales which became inextricably linked with the Arthurian legends, most notably that of the Holy Grail.