Thanks for all the feedback everyone. It sounds like I'm much better off going for either a discount premium economy ticket on the flights that this is availaible on, or just buying an upgradeable discount economy fare. Given I am flying into BNE from SIN and out of PER to SIN I would imagine the only legs I can get premium economy on are LHR-SIN and SIN-LHR? There doesn't seem to be an option from the UK Qantas website to look up Premium Economy fares for some reason though? I guess I will just have to call up Qantas over here and ask them.