Qantas already codeshare on the majority of BAs LHR-MAN flights.
However Qantas acquired the old FlyBe slots from Heathrow, but due to late regulatory approval they have been unable to operate an Oz route from LHR with these slots.
Instead of letting the slots go unused for a whole season they have employed Flightline to fly a 146 between LHR and MAN to bring passengers down for connecting flights. Whilst hugely inefficient in itself, it is more efficient than not using the slots at all, which is why it is in operation for the Summer 2 season, starting next month I believe.