Clearly improving air etc. quality is a good thing, but we should not consider that that has not already been happening. From a weekly magazine this week:- the Department for the Environment and Rural Affairs has been keeping comparable statistics since 1970. Sulphur dioxide pollution in Britain has since declined by 97 per cent. That, perhaps, is not surprising, given that the main source of it was coal-burning something which largely disappeared after the Clean Air Acts. But progress on other forms of pollution has been pretty drastic, too. Nitrogen oxide pollution is down 72 per cent, non-methane volatile organic compounds down 66 per cent, PM10s (large diameter particles of soot and other matter) down 73 per cent and PM2.5s (small diameter particles) down 79 per cent.
London's air pollution has been at 'illegal levels' since 2010 because a new target was then introduced - not because it had worsened since the previous year.
Recently increased NO2 is a direct consequence of Labour (for perfectly good anti-CO2 reasons) encouraging the use of diesel without thinking through the unintended consequences. And the recent increase in PM2.5 pollution may be directly attributable to the increase in wood-burners, against consequent on government action (in this case Tory) to reduce CO2. However, and in general, air quality even in London has been improving - I am old enough to remember London pre The Clean Air Act of 1956 - and even many years after newly cleaned indoor window sills would be black with greasy dirt within days (before the 1956 Act it was that day!).
Which is not to deny that any engine which does not burn sufficiently clean should be discouraged, through pricing if not outright ban, from being used in cities. In particular, although motorbikes are more efficient than cars in some areas (for instance producing less CO2) they are far worse in others (you cannot fit Catalytic converters on most motorcycles save the very largest) and tests have shown them to be far more polluting comparatively in many areas
If the ULEZ hits cars, it certainly should hit motorcycles too, for those not meeting the same standards as for cars.