I have now completed the return journey and it all happened as expected. At the Qantas check-in you have to go past a desk where a woman sighted my vaccination certificate, negative Covid test and Passenger Declaration form, then the checkin was as normal. On arrival at Sydney there was an announcement in the plane that unless you have all these 3 forms you have to quarantine, and if you are transiting to another state you have to check their requirements. Otherwise everything is as normal except that you have to do a rapid antigen test by the next day and day 6, then have a PCR test if either is positive. Also don’t visit a nursing home or hospital for the next week.
I did my antigen test today and then received an SMS which repeated the announcement on the plane. I suppose this was generated from information in the Passenger Declaration Form.
If I had wanted to stay in the UK for more than one night, I think they have similar requirements for getting a couple of rapid antigen tests.
So all in all it is an easy process. I was nervous about the step of getting a Covid test at the airport just before departure, so I got the test on the day before departure. In fact the testing at the airport in Sydney and Rome worked well and delivered the report in about an hour after the swab (for a fee of course). There might have been a problem if you arrived late and there is a queue, so I would still do it the day before to be safe.