If, as and when AA puts its codeshare on a bunch of BA flights (the U.S. DoT gave tentative approval on May 30) you'll be able to get to Singapore or Bangkok (from LHR) as an AA codeshare. For some reason, they're not proposing to codeshare on the onward flights into Australia.... so you'd have to switch to a QF flight from SIN or BKK - for which only 70% miles, as you know.

An alternative on BA is to pay the surcharge (A$900) to upgrade to BA's World Traveller Plus (premium economy) cabin on the LHR-SIN/BKK-SYD/MEL legs - you get 100% miles for WT+, plus a 10% bonus (i.e., 110% bonus in total). The surcharge is A$600 for one leg, A$900 for both.