bad news: SYD-LHR is a slightly more than standard charge for the upgrade on BA to WT+ (I keep remembering the numbers in AU$ - so your $450 is correct I think) - but less than the cost of two segments. AFAIK only BA codes can ticket into WT+
good news: BA WT+ is OK on AA (except transAtlantic to USA) and is worth 1.5 QP per mile and gives you a 10% bonus on earned miles. (I seem to remember something about a plat challenge on a WT+ ticket last year

)
potential news: op-ups in WT+ are not unheard of.....
Happy wandering
Fred