What gave the Britons strong evidence that the Roman empire was an invasion threat?

Answer 1

The tribes of southern England, particularity the Catuvellauni and the Atrebates, already knew and had evidence that there was a threat of a Roman invasion before the Romans mobilised their troops of the French coast.. The Britons knew that the Romans were going to intervene in their internal politics. 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 past of the territory conquered by the Romans.

Answer 2

On the coast of Briton, near what is present day Kent, Roman warships could be seen scouting the coast of Briton.