Welcome to AFF Mr N, the domestic experience is very different to international especially the A380. A few other suggestions for you, some of these are QF online only.
Check in online 24 hours before your flight and select seats either window (you have to get over other people to get out of your row) or isle easy access (will have to let others out). Window has the benefit of a view, isle the benefit of easy access. Suggest the four of you get 2+2 (seats ABC, or HJK sides) rather than seats D-G 4 uness you have very little kids. You can move rows if you dont like where you are or just for a change in scenery, I am guessing its 2 adults and 2 little people, depending on how little they may want to sit together and the 2 adults can hold hands
Buy a bottle of water to take on-board or an empty bottle and fill it, they provide water but I like more than that, they also have water in the self-serve bars as well.
Get / Ask for pillow, blanket, you will get an eye mask, ear plugs are in a little pack you will have on your seat.
Select different meal types if you have a preference, e.g. veg. Dont drink coffee, too much caffeine.
Get to the airport early, suggest about 3 hour before is usually fine, there is nothing worse than panicking if you are running out of time at check in queues. Relax as much as possible in the departure area before you get on your flight, both Syd / Mel have good departure areas, past emigration. Other may not agree?
When you get to Singapore (I guess?) but anywhere, take a walk, gets the circulation / muscles working again and tires you out for the sleeping leg, dont just sit next to the depature gate.
Take a good book, magazines or other things that you enjoy doing whilst relaxing. Read the current liquids, hazardous materials etc. just in case you want to take anything unusual on-board.
Read some of trip reports on here for Y / econ flights. There are a number and it will help give you an idea what to expect.
Drugs, many on here are not a fan of taking medication but I do, not always, but to LHR I suggest that you do. Have a relaxant as well as a mild sleeping tablet. Sleeping is the best form of relation.
Write a trip report to post on here, we are always interested and add your tips as well. Usually these trips are an enjoyable experiance, relax as much as possible, don't worry about the things you cannot change you will get there. Once you have landed read the signs at LHR to know where you are going but most people will be heading the same way.:arrow: