I see your wife's application was submitted at the end of May, Jonny ... in WHICH case - all things being equal - she SHOULD have her visa in sufficient time for you to travel together on the pre-booked Davao/Singapore flight, i.e. if the British Embassy maintains its current target-schedules.
However, in the unlikely event of these NOT being met, I found Singapore Airlines to be very flexible when it comes to changing dates. And here again, I speak from personal experience in that, due to prevailing circumstances at the time we were awaiting the processing of MY wife's visa, I'd to postpone my homeward-bound flight. But, much to my relief, the airline office in Davao accepted my reasons for this, and allowed me to travel on the later date I mentioned in my reponse to Ross.
it was well-worth taking a chance on a fare like the one you earned through [?Krisflyer] airmiles; it would've been daft not to! So let's continue to look on the bright side ... and good luck to you both with your application.