That's exactly what is expected of you if you score a staff opup (at least for us non-BA employed, BA staff flyers). What you *actually* do is down to how respectful you are, but bad behaviour can cause your interline access to be removed. I was flying from the Far East to London a few months ago on a staff flight and was due to work next morning (covering unexpected sickness). I asked if I could use a flat bed in the practically empty J cabin to get a few hours sleep, and the crew were happy to oblige. I certainly didn't expect any preferential treatment, took whatever meal was given to me and didn't put any extra workload on the staff.

The official line is you get the cabin you book as a staff flyer, and if the loading dictates that your cabin is full, a paying customer (Avios or not) is bumped up a class to make room for you. Once on board it's down to the CSD whether or not you are upgraded, and it does happen, but usually it's on quiet flights, or flights where the equipment for the seat you are sitting on is broken. Sometimes as a staff flyer you get the broken IFE, shoddy tray table or non-reclining seat.

As a side note, most of my flying is paid Y/J, but for super short notice flights the cost is exorbitant and/or there is no availability. You buy a standby ticket, go to the airport and wait. The stress involved in that should not be underestimated. I've also taken flights sat on a jump seat, both in and out of the cockpit, the former being great, the latter horrific on a LH flight.