Can I return a used car to a dealer?

If you bought from a dealer and the car genuinely doesn't work properly, doesn't do what it's meant to do, or doesn't match how it was advertised, you have a short-term right to reject it within 30 days for a full refund.

That's different from simply changing your mind. There's no general legal right to return a car just because you've decided you don't like it after all, unless the dealer's own policy specifically allows that, or you bought it at a distance, for example entirely online without seeing it in person first, which carries its own separate set of rules.

If it's past 30 days, you still have rights, they just shift towards a repair or replacement first, rather than an automatic refund.

Source: legislation.gov.uk: Consumer Rights Act 2015