I have used BA business class to Sydney. To get a fare in the region of £1900 instead of £3500 I used Oslo. Last time I booked a one way fare LGW to Oslo with Norwegian for around £50 and had another ticket OSL-LHR-SIN-SYD & return on BA. On the way back I asked them to only check the luggage through to LHR as I had a full days layover, I then felt rather tired and emotional and did not use the last sector LHR-OSL. I understand this way of saving money works on lots of different routes, but worth shopping around for different fares from points such as OSL-BGO-AMS-BRU-DUB.