I used to live in the Midlands (whole childhood), and moved to Scotland with work in 2003. A few of your comments jumped out at me.
I'd love DC to grow up speaking Gaelic, playing traditional Scottish music etc etc. It's hard to put the value of this into words, but it's very important, and DH recognises that it's something Sydney can't offer
No idea what that school in Leith is like, but other schools will not be pushing Gaelic whatsoever. When we moved here (just outside Edinburgh), my children were aged 4 and 5. They are now 24 and 25. Neither one can speak any Gaelic, neither can any of their friends, and I can honestly say, that in the 20 years I've lived here, I don't think I've ever heard anyone speak it. Yes, they will have traditional Scottish things in schools, and the chance to play bagpipes or do Scottish dance, but the kids who took that up were few and far between!
We're able to afford a flat, the same size as our current house, in the centre of Edinburgh, on a mortgage small enough that I wouldn't need to work
But why not explore areas that are commutable, where you get much more house for your money? We now live in Dalgety Bay, which is just across the water. I can see Edinburgh and the Forth Rail Bridge from our windows. If we want a day in Edinburgh, we get the train from Inverkeithing, that takes 20 minutes right into Edinburgh Waverly. The station car park is free. We have 10,000 people in our town, it's very, very family focused, with excellent primary schools, some lovely restaurants, a Community Council that arrange Gala days and the like, a bowling green, a sports centre, and the houses are so much cheaper. We are in a 5 bed house with sea views. In Edinburgh it would be at least 5 times more expensive. We can drive to Edinburgh Airport in 15 minutes, or park up at the Halbeath Park and Ride, leave the car there (free) and hop on a Jet747 bus to the airport.
It's freezing
OMG, no it's not. It's cold now, but it's December. In the summer, we had weather so hot that the Vets were telling people not to walk their dogs, as dogs were collapsing and the pavements were burning their paws. We sat outside for a pre dinner drink most days at about 5pm, from April through September, and it was glorious.
Cant speak for Sydney at all, but have heard some Australians say that it feels isolating there? Less easy to travel from?