I was T&GWU convenor for Birmingham (Digbeth - although actually being run from the Edgbaston St Bus Station at the time) for five years c1987-92. When I joined the company in 1973 it was a 'closed shop' (you had to be in the union) but later this was made illegal, still most joined or stayed in.
In the early days of unionisation it was tolerated, and the company even provided small 'cabins' for the union collector to stand in and collect the weekly fees from members - later (in my time) there was office space available.
I found it an interesting position, having authority equal to the depot manager but without most of the ensuing responsibility, and found the task of helping others (not always with work-related problems, either!) fascinating and satisfying. One day I should write my memoirs on it!!