Having grown up in the London Transport land of the 1950s and 1960s I always had an expectation of how a driver (or conductor, for that matter) dressed, even the younger ones in the '60s who began to push the limits but never broke them. For instance, although by that time drivers were permitted to drive in (smart) shirtsleeves no conductor was allowed this luxury, and I remember my shock at seeing a London conductor jacketless for the first time in the late '70s or early '80s.
Down here in Cornwall, in First land, I got used to seeing drivers wearing their hi-viz jackets when driving. but never felt it gave the best impression of the company, so I was glad to read an interview in CBW a few years ago with a First high-up who agreed with me on this, and I have to say these days it's a lot less prevalent.