I’ve done London to Sydney lots of times with my DC, pretty much annually from when the youngest was 7 months old (except Covid year). Thankfully they are now 9 and 13 and seasoned flyers! My tips:
Spare clothes for you and your DH as well as the DC in your carry on luggage. Chances of being thrown up on are high, as well as drinks food, etc being spilt in you. Be careful not to go overboard with snacks as it increases the chances of being thrown up on. And keep the throw up bags close!
My DC hated the airplane meals, so although you should order the kids meals, I’d take not just snacks but enough food for the whole flight of stuff you know the toddler will eat. Eg we stocked up on Pret ham and cheese sandwiches at Heathrow. Still do, actually, as DS age 9 still won’t eat the plane food.
DH and I would take turns letting DC nap across their own seat and ours and stand at the back of the plane for hours at a time reading our kindles. That may be harder if you don’t have a seat for the toddler, but nevertheless I strongly recommend kindles with some very easy reading for you both, because it means that you can read with the backlit screen standing at the back of the plane in the dark even when it’s in night cycle.
Comfortable child sized headphones with an adapter to make them fit into the plane entertainment system, as little kids find the free earbud headphones hard.
Cheap tablet pre loaded with Bluey, Peppa Pig, whatever. On board entertainment is getting worse and worse (I assume airlines are cutting costs because so many people just bring their own entertainment on phones tablets anyway). Again with headphones that fit your toddler. When my oldest was little you used to get virtually every movie Pixar had ever made on the flight, the last two years we’ve all be really unimpressed with how little there was.
DC lived buying a few magazines at WH Smith at the airport.
Get to the airport massively early to make absolutely sure they haven’t changed your seat allocation. When we had a baby, we booked through Flight Centre to be sure of getting the seat with a bassinet. Some airlines will guarantee that at booking, some won’t. flight Centre will know.
Find a less busy spot at the airport to let your toddler run around a bit before boarding.
Good luck! As PP have said, it will be somewhere between tough and horrible, but either way it will be worth it!