I agree with you. A strict emissions test regime would be a good first step. But politicians have known about this for years and little has been done - apparently it's a too hard (or too expensive) problem to fix.
The thing is, even brand-new "clean" diesels are still worse polluters, for toxic particulates and NOx, compared to petrol and petrol-electric hybrid vehicles.
London already has a disincentive on diesels coming into effect by 2019, when pre-Euro 6 (i.e. older than September 2015) diesel vehicles will be subject to a £12.50/day charge to drive in London.
There are calls to extend this to cover all diesel vehicles, eventually leading to a total ban:
Licensing of new diesel taxis will also soon be prohibited in London, and all single-deck busses will be zero-emission by 2020 (and double-deck busses will, at minimum, be hybrids).
