To be wholly honest, is not quite that simple.
The new Consumer Rights Act gives a purchaser the right to reject within 28 days of sale if goods are not of a satisfactory quality.
What is of a satisfactory quality is an open question. A cosmetic fault might well not get over that hurdle. This is fairly new legislation so it is still open to interpretation.
They are offering a repair here which is likely to be considered adequate given the nature of the fault.
It is not likely that courts will smile upon attempts to reject an entire car for a cosmetic fault I'm afraid.
If the repair is inadequate though then you would have a right to reject which would be revived.
Can I clarify anything for you?