So if you look at their focus:
a) reduce pollution overall
b) reduce pollution in cities.
Both are best served by no output from engines - served by having electric only as 0 tax.
Next is by reducing the number of cars - typically the more expensive cars have larger engines regardless of petrol or diesel too.
Finally newer engines and those which are not tampered with (i.e. removing DPF) should be encouraged.
If they want to reduce city pollution then they need to impose tax on vehicles inside cities - regardless of fuel.
Petrol engines pollute with CO2 - acid rain results. Diesel engines pollute NO2/3 & carbon particulates -the euro6 such as the PSA engines claim to drop particulates and, with AdBlue, output Nitrogen only with far less CO2.
Now look at the cost of per-mile of the above (including everything) and for many (including deliveries) then diesels are more economical. However when you look at model of how most cars get used on a commute - it's something a 3 cylinder petrol engine can do.. or a hybrid could do better at the moment... and the focus of the electrical car models will be soon enough..
What is certain - is that the industry will not invest if there's no return in sales. For this reason I see that you'll still have diesel for long haul, you'll still have a mix of options in the middle and then for 'around town' the incentive will be provide electrical.
That seems to be what is happening with the government.
