In late summer, 55 BCE, even though it was late in the campaigning season, Caesar decided to make an expedition to Britain. He summoned merchants who traded with the island, but they were unable or unwilling to give him any useful information about the inhabitants and their military tactics, or about harbours he could use, presumably not wanting to lose their monopoly on cross-channel trade. He sent a tribune, Gaius Volusenus, to scout the coast in a single warship. He probably examined the Kent coast between Hythe and Sandwich, but was unable to land, since he "did not dare leave his ship and entrust himself to the barbarians", and after five days returned to give Caesar what intelligence he had managed to gather.

By then, ambassadors from some of the British states, warned by merchants of the impending invasion, had arrived promising their submission. Caesar sent them back, along with his ally Commius, king of the Belgae Atrebates, to use their influence to win over as many other states as possible.

He gathered a fleet consisting of eighty transport ships, sufficient to carry two legions (Legio VII and Legio X), and an unknown number of warships under a quaestor, at an unnamed port in the territory of the Morini, almost certainly Portus Itius (Boulogne). Another eighteen transports of cavalry were to sail from a different port, probably Ambleteuse. These ships may have been triremes or biremes, or may have been adapted from Venetic designs Caesar had seen previously, or may even have been requisitioned from the Veneti and other coastal tribes. Clearly in a hurry, Caesar himself left a garrison at the port and set out "at the third watch" – well after midnight – on 23 August with the legions, leaving the cavalry to march to their ships, embark, and join him as soon as possible. In light of later events, this was either a tactical mistake or (along with the fact that the legions came over without baggage or heavy siege gear) confirms the invasion was not intended for complete conquest.