For someone considering Sydney to London, there can be a saving of over $1k by buying 2 tickets as follows:
SYD-HKG 2,871
HKG-LHR 1,520 ( hkd12,940 )
For dates I checked in October, SYD-LHR via SIN is Aud5,472.
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I presume this is a J ticket, if so the return price would work out pretty close to the recent QF J companion fare sale.