Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
The brave and cautious prince Cassivellaunus, who ruled in what is now Middlesex and the surrounding district--formerly the terror of the Celts to the south of the Thames, but now the protector and champion of the whole nation--had headed the defence of the land. He soon saw that nothing at all could be done with the Celtic infantry against the Roman, and that the mass of the general levy-- which it was difficult to feed and difficult to control--was only a hindrance to the defence; he therefore dismissed it and retained only the war-chariots, of which he collected 4000, and in which the warriors, accustomed to leap down from their chariots and fight on foot, could be employed in a twofold manner like the burgess- cavalry of the earliest Rome.
When Caesar was once more able to continue his march, he met with no interruption to it; but the British war-chariots moved always in front and alongside of the Roman army, induced the evacuation of the country (which from the absence of towns proved no great difficulty), prevented the sending out of detachments, and threatened the communications. The Thames was crossed--apparently between Kingston and Brentford above London--by the Romans; they moved forward, but made no real progress; the general achieved no victory, the soldiers made no booty, and the only actual result, the submission of the Trinobante in the modern Essex, was less the effect of a dread of the Romans than of the deep hostility between this canton and Cassivellaunus.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=94