I don't remember it being used on (what was then) southern region (south east section) suburban trains that I travelled on occasionally from the late 70s and commuted on for a few years in the early 90s.
Similar to what
@John Webb says, this would have been on EMU sets with starter bell from guard to driver, which (I think) was usually repeated by driver on the driver / guard intercom bell (loudaphone?) - I remember one or two journeys where drivers (or were they still officially motormen then?) gave one 'beep' on the horn before starting, but this was enough of a rarity for me to notice. Whether this was an individual driver's habit, or an occasion when the intercom bell wasn't working, I couldn't say.
I faintly remember having one or two journeys with my late father on 4 SUB units shortly before they were withdrawn, and it being pointed out that the guard used a green flag or lamp to give a starting signal on them, but can't remember if or how the driver acknowledged the starting signal.