Having purchased an N class ticket through an agent, I got my ticket upgraded to WT+ (T Class I think, but whichever is the lower) on one of the legs for about £200, the premium does seem to be £200 per leg, so £800 from London to Sydney.
However, the flight was busy, and I did get moved to Club World, and for some obscure reason, this happened on board!
If you booked these tickets through an agent, it might be worth your while talking to them first to find out if there's availability.