I got my first car two years ago, a 2012 Peugeot 207 for less than £2,000 and it's ULEZ compliant. You can get ULEZ compliant cars even cheaper than that too, especially considering cars are extremely expensive where I live vs other parts of the country.
ULEZ has already improved the air quality in London by a noticeable and measured amount. It's also not a tax on the poorest, if you can afford a car then you can afford to run a car you can afford one that's ULEZ compliant.