It will be hard to stay on BA metal based at SIN except to/from LHR. You'd mostly be connecting through HKG on CX, plus using QF to/from Australia to stay with oneworld.

Of course SQ is better than BA. For starters, they don't have the ridiculous policy of charging nonelites for advance seat reservations in business class. FAs are generally wonderful and want to offer good service, there's Book the Cook, TPR is wonderful, the premium cabin/elite check in lounge and private passport control exSIN is wonderful, etc.