I was asked at check in desk at Heathrow for date of return, flight and duration of stay. They didn't need the TDAC because it can also be done at airport on arrival.
I have a ticket says I depart from UK. The TDAC page for Q&A says to put country you depart. I've done it and submitted. It's been accepted and given me a qr code.
They don't ask for the flight number of your departing flight. They ask what country are you coming from. The flight number only relates to the plane you got off. I've done mine UK departure, and my flight number for the plane I got off. It was accepted.
I've just completed mine. What country you're departing is Aus. It's up to you if you want to say you're coming from another country. . The flight number is so they know what plane you got off.