I think the OP means fly LAX-YYZ on AA then connect to YYZ-LHR.

One hour is really tight, but if it is in one PNR and it lets you do it (I think this falls within the min connecting times) then you are (technically) fine. If your plane is late, be prepared to be bumped to the next YYZ-LHR BA flight (which may be in the back of an old 744, you didn't post what day you were flying so the 787 may be the last out of the day).