Which of the following historic groups invaded Britain first?

In 55 BC (Before Christ), Celtic Britain was invaded by the Romans under Julius Caesar. Caesar's two invasions did not conquer Britain but established it as a major trading partner of Rome.

A century later, a botched attempt to conquer Britain was made under the emperor Caligula. Caligula's uncle and successor, Claudius, was the first emperor to oversee a successful invasion. He used as an excuse the pleas for help that came from the Atrebates, Celtic allies of Rome, and landed an army near present-day Richborough. The initial landings were unopposed, and the Celts delayed in responding to the invasion. When, under their leaders Caratacus and Togodumnus, they did, they were too late and were defeated in several battles, most notably that of the River Medway.

Claudius arrived himself, bringing up to 38 war elephants with him. When the Celts were finally defeated and Caratacus forced to flee to Wales, Claudius returned to Rome.

The Germanic invasions started in 410

The Viking Invasions started in 793.

The Danish invasion started in 1069.