Welcome to Flyertalk and the Singapore Airlines Krisflyer forum, Mokington.

Doesn't matter if its VS ticket, but the flights in question must be on SQ flight number and not VS codeshared flight on SQ.

If you are booked on the usual booking classes of Y, B and E (on SQ flight number), then upgrades are possible. As the flights between LHR and SYD are operated by mostly 77W, and A388 flights, one must know the astronomical amount of miles needed to do such an upgrade.

You need 400,000 Krisflyer miles to upgrade ONEWAY from UK to SYD!

http://www.singaporeair.com/saa/en_U...ess_Awards.pdf

Anyway, doubt the fare between LHR & SYD would have been as high as you said. It could be one fare class is not available and the computer automatically chooses the higher fare class. You can and should avail yourself to the services of a travel agency who can advise and book you on the fare that you want.