As an ex BA engineer, I know the flight crew are expected to share headsets - in fact, we made sure of it by bolting the cables into the aircraft to stop them being removed! However, BA do supply a bag of antiseptic wipes on the flight deck to wipe the headsets, telephone handsets, and oxygen masks before flight. Before they introduced the ANR headsets, they even supplied spare foam mic & ear pads on the flight decks, for the same reason. Not sure if this was specified or written anywhere, maybe a BA driver will be able to help with more info?