Good luck trying to tie everything up in the UK, we did find that bit all very last minute.
I don't envy you, for your journey, by making ours just London to Sydney initially, and then the move the Canberra later. I guess you will need to do London to Scotland, then 2 legs to Sydney? and then on to Canberra.
Anything that will help your little ones settle for some sleep on the flight would be a good idea, whilst accepting that they will want to watch an excessive amount of in-flight entertainment tv shows and spend some of the time running up and down the aisles amusing staff and other passengers! We found that our son didn't have great need for the activity books we brought, a couple of things was enough, because the plane was so exciting. At least with the 2 year olds they'll have their own seats (I know there's a cost to that).
Think about what you need for different stages; what do we need on the plane, don't put other stuff in hand luggage as you'll need to haul it through at least 3 airports, what will you need when you get there and is it cheap to buy on arrival.
Oh and fill out the immigration forms they give you on the plane in good time (when it's light and you're not trying to sleep) rather than have a rush with them when you get to Australia.
Are you planning on grocery shopping on arrival day, or can you consider doing an online shop with coles or woolworths in advance to be delivered once you arrive?
Andy