Call BA and tell them you found it cheaper but can’t book it online and see if they can assist. I found a great deal online but got an error message when processing payment so I called to see if they could take the payment over the phone. The agent wasn’t able to honor the AARP fare I found (which was about 10% off the standard published fare) but was able to put something together for 20% less than what I had already found!

Worth a shot to see if they can put something together for you!