Am I right in thinking that these mixed class fares can only be booked by a travel agent? I can’t find anywhere that I can book these online, or any mention of a mixed class fare close to the $3K mark. Have you got any additional hints for me? I’ll continue the search.
mixed class can be booked yourself, but it involves two tickets, one ex Australia, the other ex Asia. Deals ex Asia change almost daily, but the 'regulars' in the $2000-2500 range include Sri Lankan (ex bangkok in particular), Vietnam airlines, Qatar, Oman air, sometimes finnair. Google flights is a good starting point, mix and match origins and destinations, and check AFF's 'cheap international fares' thread. note the UK will have a much much higher tax, so if you want to save money you could fly in to London and out of Europe.
ex Asia starting points are KUL, SIN, BKK, HKG (sometimes), MNL, CGK.
combine that ticket with your choice of economy class to Asia, but as its separate tickets, leave a good connecting time (you know, not an hour to have to collect bags, clear immigration etc). or even build in a stopover if you want.
for london, buying miles through lifemiles is a good choice, with their recent 240% bonus the fare would have been ~$3110 (Aussie) plus actual taxes (no fuel surcharges). that would be on Thai airways, business class all the way from Australia, straight through on the same ticket.