Lhr to Syd is the only route BA currently fly which cannot be done non stop with their current fleet. BA will not buy additional aircraft just to cover one flight a day.

Qantas with non stop will be able to charge a premium but they do not have enough slots to get anywhere near satisfying the demand on this route. The majority of passengers will still need to stop en route. I suspect a BA flight via Sin will still be viable but if not BA will just cancel the route.