The primary issue is older engines (diesel and petrol) in towns and cities.
Copy the German Umweltsplakatte system - create emission zones in the most affected cities and towns and restrict entry to vehicles that meet a defined cut-off in terms of emissions standards. The German system is colour-coded based on the Euro emissions standard - Black (Euro 1), Red (2), Yellow (3), Green (4), Blue (5) and Purple (6). You get a sticker for your car based on the performance standard it's built to (which admittedly still leaves the VW problem). Most of the cities that have adopted the system have progressed to Green, prohibiting pre-Euro 4 (pre-2005) vehicles from entering the emission control zone. There's still one or two Yellow (Euro 3) systems. Pressure is building to move the worst polluted cities from Green to Blue or Purple where air quality still fails to meet targets. But any changes are debated, advertised years in advance and mitigation mechanisms are put in place for those least able to cope with the change (hardship extensions and disability exemptions).
The advantage of this system is that it can be progressive over time (Yellow > Green > Blue, extent of zone) and it targets the problem where it occurs without adversely affecting rural areas and drivers that don't drive into the major towns, cities and conurbations. But someone would have to stand up and announce it now for introduction 2-4 years down the line so that everyone can plan for it.
The UK has to tackle some of it's exemptions:
- New black cabs aren't required to stop using diesel and adopt hybrid technologies until 2018 and existing cabs can be run until they expire.
- Stationary engines (generators) and construction plant needs to be brought into line with emissions control legislation. Crossrail made a start on this but only required Euro 3b compliance.
- Buses need to be addressed, although cleaner per passenger mile than a fossil fuel car they are a significant problem and only around 20% of the TfL fleet are low or lower emission.
Which reminds me, I need to apply for an Umweltsplakatte (Blue) before my next trip.