Not strictly true, I've spent the past 39yrs working on European cars. The paint finish and protection was better and they used a slightly thicker gauge of steel than the mass produced offerings. Volvo were also one of the first to use wheel arch liners. I've certainly seen my share of corroded Swedish vehicles.
In the early days many of the components that Saab and Volvo used were UK sourced anyway. Ford and Vauxhall used thinner sheet metal to save weight and cost and make the fuel consumption attractive to fleet buyers.