After I copied all cars specifications from Parkers.co.uk into Excel format, I analyzed it to show you how cars sold in United Kingdom evolved over past 40 years. I calculated using AVERAGEIFS formulas. Charts made at 16 September 2019 update with 83430 model versions. You can buy Excel database: UK Car Database.
In case of American car database, there is a row for each “Model Year” a car was sold in, so each year calculate average data values of cars available that year and the evolution looks constant. But in case of UK car database, the statistics are based on the year of launch, so there are large variations in average data values from one year to another, only after 1990 number of cars launched per year became high enough to reduce variations of average values, but still do not make a constant evolution, this depends if in a particular year were launched more small cars or more big cars.