Why did the Romans invade Briton?

The reason for the invasion is nothing to do with trade. Caesar was fighting the Gauls and he was concerned that support would be received from the British tribes who were ethnically connected. Further, Julius Caesar did not invade in 43AD (he as long dead). This successful invasion was lead by the Emperor Claudius. In his case, it was for glory. His hold over Rome was tenuous and he felt that a conquest would bolster his power - which it did for a time.