How did the Celtic tribes of Briton know that ancient Rome was planning an invasion?

Answer 1

Knowledge of the plans for a Roman invasion involved more than just warnings by the Gallic traders. It was predictable. The Britons of southern England interacted with the Romans for decades before the invasion. This included trade and diplomatic relations. In late 39 Adminius was exiled by his father, Cunobelinus, the king of the Catuvellauni. This prompted the emperor Caligula to prepare an invasion of Britain, which did not materialise. Later the Catuvellauni tribe defeated the Atrebate tribe and Verica, the king of the latter, was exiled. This is what either prompted or gave Claudius a pretext to invade southern Britain. The Catuvellauni knew that a Roman intervention was likely. They knew that the Atrebates were likely to seek Roman support. Cogidubnus, who is thought to have been an Atrebate, supported the Roman invasion. He became a Roman client-king in part of the territory conquered by the Romans.

Answer 2

Early in the first century AD, the Celtic tribes in Britain became aware of the pending Roman invasion of their land. Gallic merchant traders warned the Britons that a Roman army and fleet was organizing on the northern coast of Gaul near Bononia. Also, visible confirmation was attained as Roman navy scouting ships could be seen from southern Briton.