The UK accepts PCR or RAT for the pre-departure test. When I was at the Sydney checkin last week, the QF staff didn't know that, which caused chaos in the queue in front of me, with people having opted for the cheaper RATs. So if you have a PCR test, I think that's safer. Then Histopath has RATs with a 30 min turnaround at the international terminal if you're desperate.
On the UK end, they will check that you have completed your passenger locator form. This includes the reference number for your booked Day 2 test. The Day 2 test has to be a PCR. QF checks these on the Sydney end as well.