The European legal machinery is closing in on the London government's disregard for the air we breathe;
http://curia.europa.eu/jcms/upload/docs/application/pdf/2014-11/cp140153en.pdf
Transport emissions are a large contributor to urban NOX. This is a judgement that cycle campaigners might to well to flaunt, knowing that UKIP will wish to flout it.
I suppose that given that relations with the EU are a reserved matter, our government will not have any direct involvement in this affair? Presumably they could attempt to comply with the limits locally using non-reserved powers? That might in turn be a powerful propaganda tool.