Rule 79:
Except when engaged upon the collection of fares, answering enquiries and the performance of other such duties conductors must be on the platform of their vehicles, ready to assist passengers and on the alert for intending passengers. (LT Country Buses & Coaches Rule Book, 1967)
A rule clearly designed for open-platform double-decks. There wasn't really a "platform" as such on the RF single decks so I would suggest that rule was open to interpretation!
I commuted to/from school on a Green Line in the 1960s and I seem to remember the conductor would usually stand in the staircase well by the door, at the front beside the driving cab or, as
@Jon_jpwh says, sit on the single seat behind it at quiet times.