Look at cars of today vs. those of a decade or more ago.

Count the number of things that have been become de rigeur or even mandatory since, the failure of any one of which will ensure that a cheap car is written off as the parts to fix cost more than the car's worth, before you include labour charges(!)

e.g. ABS, ESP, variable-vane turbos, etc. The list is as long as your arm.

Even some fairly ordinary bits have the same effect. Consider a ten plus year old car worth a few hundred quid. A 100 quid clutch job is probably worth doing, but add a DMF to that and most will get shot of the car.

Time was that you could keep an old clunker going for as long as you were prepared to fettle it at minimal cost (usually until either the engine expired or terminal rust developed). No more.

I understand the "logic" behind scrappage, ie not cost-effective to repair, but has the average really changed so much in such a short time ? Or is the reduction artificial due to the scrappage incentives of a few years back ?

I think a car's life mostly reflects 'fashionability'. After 12-ish years cars which do typical mileages are starting to need repairs more often, and are usually worth £1000 or less, largely because most owners want something more trendy. They are worth little because supply exceeds demand, so many get scrapped despite being rust-free and roadworthy. Usually total waste, really.