The Triumph of Claudius

Claudius's invasion in AD 43 under Claudius was unopposed. The British army, if it had previously gathered to fight them, had simply gone home. Initially they were subjected to a guerrilla campaign rather than any sort of head on fight. Romans are an elite force and anyone who meets them in battle is very likely to be beaten by them. When the Romans beat you in battle they slaughter large numbers of your warriors. They'll take slaves, they'll devastate the area, they'll occupy you. It's not a process you want to go through lightly.