I'm sure you've done everything you can Laura, and it's true that emigrating doesn't work for everyone, and you cannot live on sunshine alone. I really feel for you, and think you are being really brave to admit it's not working and look at the other options - so many people bury their heads in the sand until it's too late, so you can hold your head up girl, there's no shame in admitting it didn't work, at least you gave it your best shot and tried it, rather than spending your life thinking 'what if?'!
Just one thought (and sorry if it's something you've already considered and rejected for whatever reason) but have you thought of looking somewhere outside of Adelaide in one of the regions - Barossa, Murray Bridge, that sort of place (so not TOTALLY cut off from civilisation!)? I think a lot of the time when a profession is on the 'demand' list that means that they are short of people 'out in the stix' a bit, rather than in the more populated centre. You might also find that there is more community support somewhere like that, less need for a car for your OH, and still the 'lifestyle' for your kids. I have been speaking to a lot of people out towards Clare this week, admittedly in building type trades, and they are all crying out for people, so perhaps it might be worth a few phone calls to places further out, where rents may also be cheaper.
As I said, you have probably pursued every avenue so if you still feel going back to the UK is the best thing for your family, then go for it while you still can, and give yourself a pat on the back for having tried it in the first place, and having the courage to go back.