Moderator, Iberia Airlines, Airport Lounges, and Ambassador, British Airways Executive Club
Join Date: Feb 2010
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Posts: 63,856
I don't know about Main Cabin Extra aspect, but I do know that yes if you buy direct from BA then it will get 3 Avios per £1, and I've done it a few times. The big disadvantage is that when AA change your flight (note, not if) then if you book with AA a quick call will allow you to be shifted to anything vaguely sensible. On BA it is theoretically the same but it getting agents to do this sometimes requires a modicum of patience. It is necessary to wait until fully ticketed before you can select seats on AA unless it was done during the booking process, and don't forget to put your Known Traveler information in at that point, if relevant, BA can't do that.