No, the Casse didn't, but they did conquer southern Britain and had a monopoly on tin trade in northern Europe, and controlled so much tin that traders as far as the middle east went to Britain to buy it cheaply (it was so cheap it offset the expense of the long trip and STILL saved them money). They also sheltered Gallic leaders and supposedly provided soldiers to Gallic revolts (which Caesar used as an excuse to invade Britain). They fought the Romans in the period when Caesar invaded, and at the time, controlled all of the south of Britain (and Cassivellaunius was recognized as 'king of Britain', though they called the Casse, at the time 'Catuvellauni', which is something like 'Kingdom/Land of Vellaunius').

They were selected in part because they are a trading power, and also because of their political importance to the Gauls. Like the Aedui and Arverni, the Casse were inheritors of Ambicatus's former great Gallic kingdom. They did not seem overtly interested in recreating it, though they did support the Aedui at times, suggesting that they were supporting the legitimate claimants. Further, they had the most advanced military in Britain, and British armies are unique enough that it would be hard to represent them as just some roving rebels; at the same time, remember that RTR once said that all Celts are basically the same, which is laughably false; many of them didn't even consider eachother the same people (though the Casse, Aedui, and Arverni did; they were all Gauls, as the southeast of Britain was Gallic).