Elton ('Origins of English Hist.') conjectures that these tribes did not migrate to Britain till after Caesar's day. But there is no evidence for this, and my view seems better to explain the situation.
Solinus (A.D. 80) says of Britain, "alterius orbis nomen mereretur." This passage is probably the origin of the Pope's well-known reference to St. Anselm, when Archbishop of Canterbury, as "quasi alterius orbis antistes."
A Roman legion at this date comprised ten "cohorts," i.e. some six thousand heavy-armed infantry, besides a small light-armed contingent, and an attached squadron of three hundred cavalry. Each of Caesar's transports must thus have carried from one hundred and fifty to two hundred men, and at this rate the eighteen cavalry vessels (reckoning a horse as equivalent to five men, the usual proportion for purposes of military transport) would suffice for his two squadrons.