Very cheap open-jaw flights to Sydney during the cricket world cup in March 2015.
Fly from Barcelona and return to London. Check SkyScanner for cheap on-way flights to Barcelona to start your journey.
Available in March 2015
Barcelona – Sydney: 2 March
Sydney – London: 16 March
London – Barcelona 19 March (of course if you want to stay in London, do not board this flight. You will have no problems regaining your luggage as this flight is 2 days after your arrival in London)
Singapore (it is possible to stay in Singapore for a few days. Simply edit the booking accordingly)
Use Skyscanner to find budget flights from your location to the appropriate airports.