I think I understand that bit but I'm unclear why you would do it in this case. I assume the OP must be in Canada and therefore it's easier to do it this way? .... rather than simply calling AA in the U.K. and have them issue the ticket.
Is this route able to be issued as an e-ticket? If so I'm wondering the same as
tt7 (ignoring the AA/BA confusion), why not call BA in the UK and have them issue the ticket? Yes, it's an international phone call, but for a few dollars (or less if you use VoIP) you avoid the hassle of having to educate the airline reps.