During his conquest of Gaul Caesar invaded and effectively destroyed the Veneti and Belgae tribes who had close cross channel trading links with various British tribes and according to The Gallic Wars were allied with the Veneti and aided them in said Gallic wars. Caesar's expedition to Britain was supposed to be a punitive mission with the aim of punishing and subjugating the Britons or aiding with the Gallic uprising.

Additionally its pretty reasonable to consider any forign army that turns up in your land without your expressed permission in advance to be an invading force.