Eastern Peddler …
Ask (twice) at LHR. First try at check-in where they can phone through and see if CW to F upgrades are available. I’ve even bought one for LHR-IAH at £499 in an otherwise completely un-sold F cabin, so low occupancy isn’t a block to them doing this and with 90 minutes before flight they managed to load full F catering too. They just drafted in two F-trained cabin crew from the rest of the gang to look after us who apologised when occasionally they had to go and hurl chicken curry at the Y cabin.
If this works, fine, if not ask again at the BA information counter airside. They have a bit more authority than check-in personnel.