If you are booked on BA16 SYD-LHR you will only be able to upgrade if there is availability on BA16 SYD-LHR. If there is only availability on the SYD-SIN segment you cannot upgrade unless you change your current booking.
If you are booked BA16 SYD-SIN (with either a stopover or connection to BA12) then you can upgrade.
Shame
Here's hoping for 2 seats in F just before we fly home