FROM DOT TO DOMESDAY
  Roman Britain
The importance of British grain becomes apparent in references to, the Caesar, Julian's (Caesar 355-360, and then Augustus 360-363) campaign against German tribes, who had been causing considerable devastation in Gaul:
"He decided, however, that before engaging in hostilities one thing above all imperatively demanded his attention. This was to enter and recover towns long since destroyed and abandoned, and repair their defences; also to build granaries in place of those which had been burnt, to store the corn regularly brought from Britain."
Ammianus Marcellinus (published c.390)
"In earlier times corn was shipped from the island, first over the sea and then up the Rhine, but since the barbarians had become a force to be reckoned with, they had blocked its transport and the cargo vessels had long been hauled ashore and had rotted away. A few still plied, but since they discharged their cargo in coastal ports, it was necessary to transport the grain by waggon instead of by river, and this was a very expensive affair. Julian therefore revived the practice and considered it a serious matter should he not put the carriage of grain on its former footing. He quickly produced more ships than before and examined the ways the river might be opened up for corn."
Libanius (d.393) 'Oration XVIII'
"When Julian invaded enemy territory and the Chamavi begged him to spare it as though it were friendly territory, Julian agreed... without the cooperation of the Chamavi it was impossible for grain from the island of Britain to be transported to the Roman garrisons - he was induced by necessity to grant them peace, demanding hostages as a surety of their good faith."
Eunapius (d.c.420) 'Fragment 12'
"As the Rhine discharges itself at the extremity of Germany into the Atlantic Ocean, and the island of Britain is about nine hundred stadia [approx. 103 miles/166 kilometres] from its mouths, he cut timber from the woods on the banks of the river, and built eight hundred small vessels, which he sent into Britain for a supply of corn, and brought it up the Rhine. This was so often repeated, the voyage being short, that he abundantly supplied those who were restored to their cities ..."
Zosimus 'Historia Nova' (probably written in the early 500s)
Julian himself, in his 'Letter to the Athenians', says:
"Then followed the second and third years of that campaign, and by that time all the barbarians had been driven out of Gaul, most of the towns had been recovered, and a whole fleet of many ships had arrived from Britain. I had collected a fleet of six hundred ships, four hundred of which I had built in less than ten months, and I had brought them all into the Rhine, no slight achievement, on account of the neighbouring barbarians who kept attacking me."
Translations:
Julian by Wilmer Cave Wright
Eunapius and Libanius by S. Ireland
Ammianus Marcellinus by Walter Hamilton