Well one thing to bare in mind is that the 43 AD/CE wasn't the first time Roman had attempted to do this. At the tail end of the Gallic was Caesar made two invasion attempts against Britain in 55BCE and 54BCE.

During this first invasion even the existence of the Romans would not have been a surprise for the Britons. They certainly traded and had contact with the Gauls of continental Europe. During the Gallic wars as Caesar spread Roman influence northwards into central, western and Northern Gaul information and stories of the Romans would have almost certainly reached across the channel as a result of this, Caesar goes so far as to claim that the Britons allied with and assisted certain Gallic tribes on the continent which was the justification he gave for his invasion. We know that in the lead up to the invasion Caesar for example gathered local Gallic merchants to ask them what they knew about Briton and ahead of the main invasion began scouting the coast of Briton a little.

When Caesar set off he crossed the shortest stretch of the channel, between near modern Calais to near modern Dover but even that short cross caused problems with Caesar's cavalry failing to cross the channel with him due to the weather. When he arrived on the other side Caesar says he found a small army of Britons managed to assemble to face him before he got to a place he could land, perhaps having been tipped off to the threat of the invasion by the Gauls. This force was able to keep up with the Roman troop ships as they moved parallel to the coast looking for a good landing spot, the Romans being forced to disembark their vessels in deep water and wade ashore exchanging missile fire in the surf and picking at any who got separated from the group.

Eventually Caesar's men made it on shore and the Britons retreated back away. Caesar would later negotiate with the Britons after he'd made a fortified camp to act as an MOB. That invasion ended in failure with Caesar being forced to withdraw but he came back the next year.

the second more successful than the first at least on a tactical scale but in the long term failed to establish a permanent roman presence in the British isle and rather only forced some local tribal leaders to swear allegiance to Rome which obviously once he was gone they had no incentive for keeping (Looking at you Afghanistan lol) .

During the intervening period there remained sporadic contact between the Romans, their Gallic subjects and Britons living near the coast and the like with trade and the like.

So come 43AD when Claudius invaded Britain for the third time, this being the one that stuck while the average person might have known less about Rome the further from the sections of the coast within easy reach of the empire you go there were certainly many Britons who were familiar with the Romans.

Concerning how astounded they were with Roman tech we have nothing directly written from the perspective of a Briton who met the Romans for the first time so we can't be sure what they would have thought of such things. We do have claims that for example Claudius brought at least one war elephant from North Africa with him to Britain which Roman authors say Britons found terrifying to face in battle and unnerving at the best of times. While they might have heard of such things from the Gallic wars, Caesar was somewhat fond of them, i don't believe there are any examples of the Celts having managed to copy the design prior to the Roman invasion certainly. Most of the standard infantry weapons would have been fairly familiar to the celts, if for example things like a higher proportion of the Roman legions might have been armoured which might have been impressive and things like their ability to rapidly produce marching camps and the similar works of engineering might have been a particularly impressive feat of organisation.