Published in 02, it looks like the UK followed other European countries into Diesel
http://news.stanford.edu/pr/02/jacobsonJGR1023.html
An exert from the paper "Control of fossil-fuel particulate black carbon and organic matter, possibly the most effective method of slowing global warming"
"Since 1896, when Svante Arrhenius first postulated the theory of global warming due to carbon dioxide, control of carbon dioxide has been considered the most effective method of slowing warming," Jacobson says in an interview. "Whereas carbon dioxide clearly causes most global warming, control of shorter-lived warming constituents, such as black carbon, should have a faster effect on slowing warming, which is the conclusion I have drawn from this study. The Kyoto Protocol of 1997 does not even consider black carbon as a pollutant to control with respect to global warming."
The reason the issue of diesel versus gasoline is important, says Jacobson, is that, in Europe, one of the major strategies for satisfying the Kyoto Protocol is to promote further the use of diesel vehicles and specifically to provide a greater tax advantage for diesel. Tax laws in all European Union countries, except the United Kingdom, currently favor diesel, thereby inadvertently promoting global warming, Jacobson says. Further, some countries, including Sweden, Finland, Norway and the Netherlands, also tax fuels based on their carbon content. These taxes also favor diesel, he notes, since diesel releases less carbon per kilometer [mile] than does gasoline. Nevertheless, the small amount of black carbon and organic matter emitted by diesel may warm the atmosphere more over 100 years than the additional carbon dioxide emitted by gasoline.
In Europe and the United States, particulate emissions from vehicles are expected to decline over the next decade. For example, by 2005, the European Union will introduce more stringent standards for particulate emissions from light duty vehicles of 0.025 grams per kilometer [0.04 grams per mile]. Even under these standards, diesel-powered cars may still warm the climate more over the next 100 years than may gasoline-powered cars, according to the study.
The state of California is implementing an even more restrictive standard in 2004, allowing only 0.006 grams per kilometer [0.01 grams per mile] of particulate emissions. Even if the California standard were introduced worldwide, says Jacobson, diesel cars may still warm the climate more than gasoline cars over 13 to 54 years.
