Calawre are warriors of exceedingly great skill that inspire their tribesmen in the battlefield. They use Gallic equipment and fill in the need for more affordable, all-purpose heavy infantry among the Britons.
Celts are a collection of hero cultures, and the Britons are no different. Among their southern tribes especially, there are warriors of exceedingly great skill, who use Gallic equipment. The Calawre (Call-a-oo-re; "Champions") are not actually generally real champions, but they wealthy, and skilled. They have good armor and good Gallic swords. They are called champions more because of their appearance than anything; they actually mostly simply fill in the need for more affordable, all-purpose heavy infantry among the Britons. They are skilled, and professional, but their expense generally would keep their numbers low.
Historically, the Britons had little in the way of 'real' heavy infantry. Most often they relied on wealthy champions to fill that need. However, sometimes they did employ bands of wealthier warriors, who were not necessarily the greatest skilled, but they would be easier to mass in one place than a group of real champions, and, while they wouldn't be AS skilled, they certainly had to have a talent for combat to survive as long as they had to afford their equipment, which was likely imported. However, the Britons also were fond of using leather and bronze scale, which was made locally. These shirts would be lined under with padding, so they could be worn comfortably. Such armor grew far more popular in wake of Roman invasion, due to armor worn by some auxilia being of similar design.
Perhaps the only reliable heavy infantry available to the Casse and their wide AOR only highlights their importance. As befitting of champions, these men should be the first in the front lines when directing a massive charge so that they can take advantage of their high armor and their high lethality of their swords and also to act as an anchor for other units due to their morale boosting effect.
Alternatively, they can be used as the main front line troops against other heavy infantry like the Neitos (Gallic Heavy Swordsmen) but they are less reliable at that role.