You cannot join BAEC now until you fly on a qualifying fare; almost certainly YVR-LHR-CAI will not qualify and cannot be credited to BAEC (unless you are paying USD 4000 or thereabouts for that ticket). So your first BAEC credited flight might be CAI-LHR and you will not have 4 BA flights when you qualify for silver; in the past BAEC has been sticky about that, and may throw away all of your subsequent tier points, not crediting them towards Gold, until you do get your 4 BA flight segments (and codeshares do count). Maybe flying 2 BA segments CPT-JNB-CPT or something like that is your best solution. Otherwise you lose credit for the long JNB-SYD sector. As for extra BA sectors after you reach Gold, they are worth precisely nothing. Finally purchasing the 5th sector in Asia is an option, but might not be cost effective (something like USD 500 in A and USD 150 in L, haven't bothered to check the current pricing). MEL-SYD is usually very cheap and readily available as a separate ticket.