I believe that when single pilot aeroplanes (right hand on stick and left hand on throttle(s)) became too big for the pilot to be seated on the centreline he migrated to the left side so that the throttles/engine levers quadrant could be side-wall mounted, thus a space was created on the right for bomb-aimer access to the nose, and seat the other crew member (initially nav, then second pilot).
BOAC, in addition to the balance issue, the (single) collective lever linkage was bell-cranks and rods, and simpler if all moved in one plane - along the airframe centreline and up the mast.