The designer and/or exec who sign off on a car that cant accomodate a full size spare need to be stranded somewhere very hot or cold with a lesser diameter spacesaver 200km from a workshop.

Sheetmetal changes would be trivial to fit a spare with the same diameter.

Should be manditory for ncap 5 star rating.

Maybe, but the car manufacturers have different goals to achieve in today’s eco world we are in - that of selling a car with lower CO2 data of which the weight of the car is a major contributing factor.
Hence almost all base cars today a low weight puncture repair tube of gunk which obviously saves a good 20kgs.......and it’s that base car that is used to measure the car’s CO2 data.......

All reasonable points, but why not provision for a full size spare when there is space and no weight penalty to do so?

6:04 for a concise summary of why
https://www.youtube.com/watch?v=UlY80ICFaMQ

wheel
 

Hi like others i also wanted a spare wheel so of I trotted to my local dealer after looking on ebay and others yes came the answer price was around £130 .

Ok being a bit sceptical I asked them how it fits above the battery well when shown it was sitting in its own tray along with the car jack and tools and yes the original in flater kit still sits in its same spot it all looks very smart and tidy