Booking classes may vary from one carrier to another, but for BA "M" stands for economy class and "I" for business class in both shorthaul and longhaul.
Generally self ticketed interline staff travel is for economy class unless person being of any CIP/VIP status to BA and they might grant business class with a better onload priority. Of course all this is very relative and depends on upgrade/downgrade/left behind situations on departure. Needless to say BA staff itself, then One World staff get on first and only then the rest.
Hope this helps. This is how it was some time ago when I was doing dispatching and moving those "are you on stand-by? ... then stand-by!" (in a positive meaning of course) back on forth...
//Dispatchguy