Sub-Roman Britain (Cavalry etc)

[M. Demetrius]

The Roman army undoubtedly had specialized troop and horse transport ships.

According to Gallic Wars, in which the Britain expedition was mentioned, some of the ships were bought or commandeered from Gaul, but many others were built in situ. (I can't imagine the lumbering and work needed to build ships to order. No doubt they had more trees to work with at that time, but still -- ) So it's quite possible you're exactly right, Ron, that the horse transports might have been available from the Gaullic coasts, but even if they were, Caesar brought quite a few horses, hundreds for sure, so they probably copied the transport design when they made their foray.