The reasons our houses aren’t as well insulated comes down to climate and cost.
Climate, people don’t die from extreme temperatures here like they do in places like Europe and North America.
Cost, we don’t have the demand for better thermal protection so anyone who wants to improve their home has to pay.
I’m in the process of trying to improve my home atm. I’ve had the blow in insulation replaced with batts, I’m replacing the windows with double glazing and I am trying to seal all of the gaps and vents that let out the heat. It will cost me 10’s of thousands of dollars by the time I’m finished, but it will make our house comfortable without costing us a fortune when we are retired.
England’s average temperature is much lower than most parts of Australia. Your population is more than double Australia’s, in a landmass significantly smaller than Australia. You live next to a continent of half a billion people who have to deal with extreme’s of cold. The Pound is worth nearly double the Australian dollar.
Given all the above, it is no surprise that double glazing is much cheaper to install in England than Australia. So most people here just put up with the 3-5mm panes of glass installed in their houses by default.