Who do I complain to about a car dealer?

The starting point is the Citizens Advice consumer helpline (0808 223 1133), rather than contacting Trading Standards directly. Citizens Advice is the first point of contact for consumer complaints, and they can refer cases on to Trading Standards where it's warranted.

If the dealer is signed up to a code of practice through the Motor Ombudsman, you can also raise a formal dispute with them once you've tried resolving it with the dealer directly. For disputes specifically about a finance agreement rather than the car itself, the Financial Ombudsman Service is the relevant body instead.

Source: citizensadvice.org.uk: problem with a used car