For every legal opinion you can find that the war was illegal, you can find one that says it was legal. for every moral objection you can find a moral justification.

at the end of the day, Britain, like Australia, went to war to prove its loyalty to the US Alliance. The justification then lies in whether that alliance is worthwhile; it involves not simply a future guarantee of military aid but current access to technology and intelligence that is not available to anyone else. For Australia's part it was worthwhile; not being British it is hard to say if it was worthwhile for Britain.