It depends very much on the crew (and you're going to have more than one crew between UK and Australia). One might be OK and another not. You've entered into a contract with the airline and I think they'd argue you're breaking it.
Personal experience. Me in Business on business, family in economy. Staff tickets and our seating was down to the checkin staff. Completely full plane because a previous flight had gone technical. I wandered back to check everything was ok and offered to spell wife (she declined, she's like that). On the way back I was accosted by cabin crew and therefore explained the situation. "How come you're in Business? We've got a Captain down the back?". "I don't know, ask check in". She backed down but there's nowt as queer as folk!