Originally Posted by
mvoight
This isn't a NH/Thai connection, so not relevant.
This is OP's return trip, which means he would be checking in at LHR when he should have checked in at DUB. I would not do this without telling the airline, and being prepared to accept a "no" response, and NOT complain if you get a "no" response. You can always PAY for the change of itinerary.
My point is that because of the way ticketing works, it's not possible to say that his ticket will definitely be cancelled.
An airline only has control over their own flights on a multi-carrier ticket; only the issuing carrier can completely void the ticket (AA in this case). If he misses his BA flight, BA is certainly within their right to cancel out the rest of his BA tickets, but short of taking control over the ticket from AA (an involved process), they cannot cancel his AA flights.
Now, if BA and AA have some method of communicating then he is SOL. I know for less affiliated carriers that communicating this kind of thing is not common (ex: between Thai and ANA or Thai and SG, alliance members but not JV partners), so it is possible to skip a segment when flying a a second carrier and keep the return itinerary intact. Whether AA and BA communicate information in his specific scenario hasn't really been established in this thread.
If I were a betting man, I would say that his AA flights won't be cancelled. But I wouldn't want to rely upon that.
Edit: I see you are checking luggage. That significantly complicates things because you'll have to speak to an agent and explain why you now suddenly have bags when they should have been checked through from DUB. If you can use an automated machine to print baggage tags you'd definitely want to try that route, or just not try to explain at all why you have bags.