Take a look at this: heycar.co.uk/guides/best-cars-that-are-ulez-exempt

Before you put any money down on a car, make sure you check on the TfL site that it's ULEZ-compliant and — as always with a used car purchase — I'd recommend making sure the car has a full service history. Give it a decent test drive and check out the MOT history (you can do this for free online here: www.gov.uk/check-mot-history), too.