I don't think it could have been either of those possibilities. There are no scheduled BA 777s between LHR and HKG, nor do QF fly any 777s. Nor are there any BA codes on QF metal between HKG and SYD.
A late sub for a sick aircraft might be plausible, but I would have thought that LHR-SIN or LHR-BKK on a 777 would require quite a bit of planning at BA that wouldn't be routinely done as they don't routinely do those routes with a 777 - and it may not be possible to do it quickly.
To add to the speculation, I wonder if he thought it was a 777 because it was laid out 2-4-2 in economy, and that this was actually because he'd got half an upgrade into World Traveller Plus?