But do not do this if you have luggage checked in! They will hold it for ransom, i.e. reprice your ticket to what you actually flew; a SYD-LHR-MNL ticket will also be far expensive than the SYD-LHR-SYD ticket you bought.

Do you really need to see Manila? Sounds like you should leave it for another trip.

And a tip: a) it is always advisable to book a ticket from the airline directly, if you think or expect or know you will later be making changes. It is usually easier to deal with the airline than with some random internet-based travel agent (if you have a human TA, that is probably better)
b) if you think you are going to want to/have to make changes, then better to wait until you know what your "final" itinerary should be before you book anything. When you book the cheapest available ticket, you can't make any changes - without paying huge change fees [in which case it would have been better to wait, then pay the going rate for the journey you actually want].

You can't "nab" a cheap fare by buying a cheap, straightforward ticket early on, and then later on make changes for little to no fee.