A good story so thanks for relaying it.
I don't think anyone should ever feel 'bad' for admitting to an Ex-EU, even in the face of snotty staff.
If BA choose to charge London residents 2-3x the price for a long-haul ticket than they do to Irish/Swedish etc. residents, then they can't be surprised if people attempt to game the system.
If the DUB fare wasn't profitable for them then they wouldn't offer it, so there should be nothing wrong with taking it up.