I think fleet commonality takes second place to operational suitability, optimal commercial aspects for large airlines . They have quite number of all the types they operate and can afford to absorb the crew commonality penalty relatively easily, despite size difference I suspect systems and fd layout between 380 and 350 are not that different being airbus types so there is probably a fair degree of overlap in the training area. Also in BA they now purchase as IAG so that has to be taken into account as well