Claimed by whom? Ancient writers? Modern pop culture? The Gauls certainly were not, and pretty much no writer who actually knew anything about them described them as such. Caesar praises various Gallic armies for their discipline and order--the wildness and disorder of the Britons is brought up as a contrast with the Gauls. The Helvetii fought arrayed in close order--Caesar describes it as a phalanx, which just means they were in close ranks with good order--and marched at a slow, deliberate pace. Indeed, the Helvetii's infantry phalanx put Caesar's cavalry to flight, and Caesar notes that their shields were overlapping, which allowed his infantry to fasten several shields together with a single javelin--Caesar notes that this was the only way he could break up their infantry line. Caesar describes the Belgae as using the familiar Roman testudo to assault city walls, which requires good discipline. The Veneti of course were brilliant sailors, which is not exactly something you'd expect from a poorly-disciplined horde. And Caesar has only good things to say about the army under Vercingetorix--a mob would be hardly capable of any of the rather impressive maneuvers Vercingetorix conducted

On Roman views of the Britons, Julius Caesar had quite a bit to say on their culture of and lifestyle. In his 'The Conquest of Gaul', Caesar talks of how the Britons who live on the Southern coast (modern day Kent) were very similar to those living in the homesteads of Gaul. They ate mainly the products of agriculture; bread and meat and were led and united under a King. Inland Britain, however, was totally different and a stereotypical 'barbaric' lifestyle. They did not farm but did collect milk from livestock. One interesting point raised by Strabo was that they had milk but no cheese. The aboriginal, inland Britons shared wives between 10-12 men, painted themselves in blue woad and shaved their heads excluding a moustache. How did Caesar know this? He never went far inland (he reached just North of the river Thames) nor mentioned any Roman who did. He most likely made up all of this from hearing rumours on the barbaric Britons. Interestingly, Caesar mentions how the Britons in the region where he actually visited (Kent) lived in rather well organised oppida.

To summarise, Roman views on Britons were quite varied and most thought that they were simple barbarians (especially people like Cicero). This was simply not the truth as they had complex communities, were great farmers, excellent horsemen and had local loyalties to a king. They were by no means Roman level civilised but they weren't mindless barbarians.