The tax is based on the passenger's entire journey. i.e. The distance from London to their final destination and not just to the destination of their first flight. The tax should be the same for all passengers travelling London to Sydney no matter which airline they fly, where the aircraft stops to refuel or whether the flight number changes or not.
The only way to have the tax calculated on the distance to the first flight only is where there is more than 24 hours between the scheduled arrival of the first flight and the scheduled departure of the second flight. Any shorter time is considered a connecting flight.
If people are really keen to avoid the tax they can always fly into LHR but take the Eurostar to Paris (or anywhere else in Europe) and fly back from there. Of course, if you're flying on Qantas you end up transiting through LHR again anway, but the tax doesn't apply to transit passengers.
Last edited by ButFli; 1st Apr 2012 at 08:24.