Originally Posted by Cymro

UUA will also try to upgrade the DUB-LHR leg, whereas over the phone you can book the upgrade for LHR-ATL only.

To complete a UUA, the short-haul would have to be in an upgradeable class as this is what is driving the fare difference.

If you book and then upgrade over the phone, the cost will be less (as long as you are OK with short-haul economy).