It is surprising how many of our road accident clients report that the police have asked them if they want to press charges against another driver.
A decision on whether to prosecute is for the Crown Prosecution Service (CPS) alone. In each case the police should supply the CPS with all the evidence and the CPS will prosecute if (1) the evidence is such that they believe there is a good chance of obtaining a conviction and (2) a prosecution is in the public interest in all the circumstances of the case.
It is not up to the victim of any offence to make this decision.
If asked this question the best response is to say whether you would support a prosecution. This means giving a full statement and, if necessary, being prepared to go to court as a witness.