It’s best to set alerts on these LGW birds as 1DF doesn’t exist on some aircraft. Which would’ve been the case on yours. The GCH comment was not correct / relevant here. Possibly just blaming “systems” to appease you ?
My tactic if assigned to one of these would be book 2DF and don’t check in / download boarding pass until the last possible minute to allow flexibility to grab 1DF if the aircraft changes.
Also which airport is MEN ?