I owe this reference to Gregory the Eremite. It is Venantius Fortunatus writing about St Paul. Venantius was born about the year 530.
Transit et Oceanum, vel qua facit insula portum,
Quasque Britannus habet terras atque ultima Thule.”
[“Yea, through the ocean he passed,
where the Port is made by an island,
And through each British realm,
and where the world endeth at Thule.”]