In his Conquest of Gaul, Caesar mentions that they made a brief little foray across the Channel, just to see what was there. The Britons were a bit 'old school' compared to their Gaulish cousins, and showed up in chariots and commenced to run sorties against the troops he'd brought along. Right at the beach, if I remember correctly.

He doesn't say whether they freaked out, but they were thrown for a loop as this was by then considered something ancient and bygone. It was more they didn't know how exactly to defend against it. After a skirmish or two they got back on their ships and headed back to Gaul.