Airlines
The "best" airline really depends on exactly what you want.
BA and QF run a very high quality joint operation. All flights through SIN and BKK have both BA and QF codes. All tickets for Oz are valid both for both BA and QF (which also allows easy connects through HKG, JNB, LAX and NRT, and on other SIN and BKK flights which are not codeshared). QF also distributes most widely within Oz. But for all this flexibility (which is probably unnecessary for students just going to SYD) you have to pay a price; BA/QF is usually more expensive than anyone else, although not much more than CX or SQ. Incidentally, personal TV screens are being progressively rolled out on QF so by northern summer 2003 there should be a pretty good chance of getting them on any QF aircraft flying from London to Oz.
CX and SQ are also very high quality operations, usually fractionally less expensive (GBP 10 or 20 round trip) than BA/QF. There are fewer connecting options (ie restricted to HKG and SIN respectively, and fewer spokes from each hub). They both have a reputation for good service - legendary, indeed, although sometimes mythical.
EK is probably the pick of the rest. But I would recommend that you avoid their 777s. They are configured 10-abreast in economy (against the standard 9-abreast) and are uncomfortable for anyone of average size or more. The A330s are fine. At the moment, I think EK also necessitates two stops en route - with all the airlines above you only need to stop once on the way to SYD. EK is planning non-stops from DXB to Oz, so it's worth checking their timetables for up-to-date information.
All of these airlines have pretty standard pitch. If greater pitch is needed, consider NZ via LAX. NZ's economy pitch (at about 34 inches) is 2-3 inches deeper than the usual. But the disadvantages are a longer journey time and less schedule choice.
The other thing which could be done for big people, of course, is to fly BA in World Traveller Plus. 8-abreast in a 747 as opposed to 10-abreast economy, and 37/38 inches of pitch. Sometimes you can get add-on offers - buy an economy fare and add GBP 100/150/200/300 each way to upgrade into WT+.
The prices one can get on MH may be linked to the withdrawal of other airlines from routes through KUL (which is why the airport is so empty). The allegation made by airlines which have pulled out is that MH has for years been systematically fare-dumping on routes through KUL, making it uneconomic for anyone to try to compete. Make of that what you will, but I personally wouldn't fly them - not for the fare-dumping principle, but for what it might say about the underlying operation.
Seasons and fares
I notice that travel is for August 2003. You may want to bear in mind that there is a high season for fares which lasts until about 15 August. From about 15 August to about 15 September there is a cheaper season, then the fares pick up back to the pre-15 August level. It may be that not all airlines will do this, but it is a factor worth bearing in mind (and asking the travel agent about) when choosing travel dates and comparing fares.