British grain for the Rhine army?

[Urselius]

There is evidence that marginal land in Britain, especially in the hinterland of the Wash, was brought under intensive agriculture in later Imperial times. There are also theories that the Saxon Shore forts, or some of them, were intended as secure concentration points for agricultural produce, for storage before being shipped to the continent. You therefore, potentially, have a number of factors favouring British grain supplies for the Rhine armies: farmland more secure from disruption by raiding than the Rhineland, a secure series of storage facilities and the hugely greater economy of transport by water over transport by waggon overland.