As someone who does this regularly
- You will need to get off the plane with all of your things
- You will get a new amenity kit, PJs etc
- If a shower is important, I would go to the closest lounge. Best showers are either QR (great esp. the one for people with ability issues - which is the closest on the left as it is huge!), and the QF J showers. QF F showers have no water pressure, and BA showers the room is tiny with no ventilation and it is like trying to get changed in a sauna which sort of destroys the whole point
- watch the boards and time - as this is a open gate they literally just open the the doors and board - I have often been the very last person to board even with 20-25 mins on the clock
- annoyingly BA15 will board, and then sit for an hour at the gate as the flight time is often fast and they cannot land before 6am at SYD.
- first world problem - often on BA15 they will not have champagne available and will often just have some awful prosecco - I like prosecco but this one is best for the battery. The LPSS will come out after take off.
- security is at the gate which has the AU liquids rule, and I think DF must be picked up in Singapore at the gate, so if you bought in LHR and it is sealed it may not be allowed.
- There is a "priority" lane for security but given you have one for economy and one for premium, and any kind of status, and people just sought of drift from one lane to the other - is why I am often last.

When you land in SYD we have these annoying little passport machines (before duty free shops) that you need to use to get a card to put in the next machine which is at immigration! - as well as the biosecurity card you fill out. Find the first machine that you see, as the queues build up and up the closer you get to the immigration desks.
If you have anything to declare in SYD like food - while waiting for your bags if you see a biosecurity agent wandering around - often with a dog, flag them down if what you have is minor such as some biscuits, etc - as they can approve your items on the spot.
Take a moment to look at the biosecurity/exit queues - there is no red or green channel, no priority channel through biosecurity but there is often a massive queue near where the QF 380 (from SIN) belt is, and then no queue at the other exits - there are at least four queues, up to eight if it gets busy.

Enjoy F, I am sure you will cope...

KF