Hence my question Chocks, do QF actually carry an alternate for the 380?
Not normally unless there is forecast weather or another requirement that would necessitate it. Sometimes that may be a call made by the dispatcher at the planning stage based on assessments made from info other than official forecasts.
In the case of London or Los Angeles, normal fuel policy arrival fuel is such that Gatwick, Stansted or Ontario (as appropriate) can be used to meet forecast alternate requirements, such that in most circumstances an approach can be made at LHR or LAX with the appropriate fuel to cover those nearby alternates - legally but not with any
planned excess of comfort!
LAX arrivals can often have significant additional fuel to cover the depressurisation case en-route. On my last LAX trip (I'm Junior - it was Christmas time
) LAX deteriorated but the "unused" depressurisation fuel gave us enough to cover Phoenix reasonably comfortably from about 20,000ft if LAX didn't improve. (ONT was fogged).....we landed in LAX as the weather improved as quickly as Sydney's deteriorates!