Thank you so much for coming back. A lot of brainpower has gone into this thread, so it's good to be able to put it some use! Someone else may have a similar problem.

The bit I have quoted is I suspect the root cause of your problem, though you are entirely blameless in that respect: it meant that BA's PNR, which has a semi independent existence here, was automatically cancelled due to what was perceived to be a no-show. This is an industry standard practice. Because the fault was on the AA side, no one on BA probably picked that up until you turned up in BGO. In the ideal world, it wouldn't have happened, in this world you would have been best to ring BA and ask them to protect the booking. Not many passengers would know that, and it is largely caused by AA and BA having separate computer systems for this area, namely SABRE and Amadeus respectively, which don't talk to each other very well.

So I'm entirely sure you are right to pursue this with AA. I remain surprise that BA at BGO did not nevertheless rebook you, or at least enquire more about the circumstances. But I guess they told you to ring AA, as ticket issuer, and you then got duff information from them. A clued up agent could have got you sorted, at least on the next service, in just a few minutes.

Very unfortunate, and I hope that you get a satisfactory outcome. It's AA you need to ping for EC261, they were the operators at fault, if my theory about the PNR (inspired by Prospero) is correct. I doubt BA is at fault here, unless they were told by you or AA as to what had happened.