Manado - Davao used to be my regular visa run, and a great little place I found it. You've inspired me to check - and indeed that route still runs on Wings Air, but that's no use to use as Manado is just as impractical to reach.

The Silk Air flight I believe is the only practical solution, albeit one that leaves Singapore at unpractical times (07:25 or 09:05) - so does not connect with either of BA's flights. And unlike some SIlk Air destinations - BA does not offer a through fare.

So, for miles, it does look like HKG then Cathay to Cebu or Manila then a low cost onwards ticket.

Or...

Singapore Airlines do have sporadic sales of £650-£680 that would take in Davao - you'll have to do the calculations on how much more you're prepared to spend to get your BA Miles