Cheap Oz seats are usually sold through consolidators which have fares others cannot match.

The UK market leader in UK flights to Oz is Trailfinders (trailfinders.com). They do not have a bookable website so you'll need to call them. They sell 20% of all leisure flights to Oz so most carriers give them their best deals.

Other consolidators worth trying, who do have online booking: airline-network.co.uk and ebookers.com. Plus opodo.co.uk, travelocity.co.uk and expedia.co.uk of course.