Building a flat pack log cabin

The reality is that if you're getting a turnkey option that meets the standards required by building regulations, it's going to cost you serious money. It doesn't really matter a huge amount if it's block built, timber frame, pre-fabricated or volumetric housing— costs aren't going to diverge wildly.

There is unfortunately no nice fluffy solution where a big, safe, energy efficient home can be made to appear on-site for small money.

If you're looking to save on costs your options are to reduce the quality (by buying one of the glorified sheds the OP linked to), reduce the quantity (by choosing a smaller structure), or reduce the inputs with costs associated (by doing as much of the work as possible yourself and ideally by also roping in friends & relatives with your big sad puppy dog eyes).

I do take issue with the statement "energy efficiency regulations exist for the protection of the climate, to reduce the need for fossil fuel based heating."

While it is accurate that reducing climate change is important and energy efficiency measures are required for this, the worldwide reality is that no jurisdiction (ours included) is acting effectively to reduce emissions to the extent necessary. The "worst case scenario" climate model predictions should be renamed as "likely case scenario" .

Energy efficiency measures & renewable energy are in reality far more about improving energy security than climate protection. It is relatively easy to manipulate, massively reduce, or shut down the supply of gas (Nord Stream 1 & 2 exploded recently for example) or oil— as Russia has recently (and OPEC did in the 1970s) in wartime. It is on the other hand extremely difficult to manipulate, massively reduce, or shut down the supply of sun, wind, tidal or geothermal energy....