Just a footnote: What is referred to as Trad Jazz in GB falls into two distinct camps. In the main, the Chicago style dominates, where after playing the melody a couple of times ensemble, the front line and piano take turns to solo before returning to the main theme again. The true New Orleans style, sometimes referred to as Creole Jazz, requires much more effort as the band plays ensemble a lot more.
While there may be formal arrangements, usually the band improvise their lines from experience. This mode of playing tends to break down as the number of players increases, and for more than eight players a band tends to need arrangements to avoid tonal dissonances (there's a polite phrase!) - below that number, there is no need for arrangements if the players are accustomed to the style.