Economy passengers are not entitled to use the business class toilets - all it is is that there is no actual legal prohibition to it, unlike for flights to the USA

There is nothing that prohibits an economy class passenger on Qantas on a flight to UK from using the business or 1st class toilets, however such actions are likely to be addressed by cabin crew

I will say that AA cabin crew can be amongst the worst and some think that they are there to read magazines and so there are some that would not care much if passengers did use the wrong toilets