Think you're getting confused between the rebreather bag attached to an O2 mask (i.e. on the drop downs- some have/some do not) also on some portable bottle masks and an Ambu-Bag... semi-rigid large "zeppelin" shaped bag used as described above by medical professionals during resuscitation. (attaches to the artificial airway inserted)
Not sure about BA but most airlines have this onboard for medical professional, not cabin crew, use.
FYI for those who were asking the "SCCM/purser to the flight deck" call not only summs that person but lets the other crew know to get to a station to answer a cabin call giving more information. I.e, stop the service something important is happening. Usually this would also tell us to secure the carts/galleys in readiness for a return (but not always)