Bjohn, the police have a duty to investigate. The Crown Prosecution Service have a duty to examine the evidence and decide a) whether there is a prima facie case b) if there is, whether there is a reasonable prospect of getting a conviction ( a rather higher standard than whether, in theory, there's a case) c) whether it is in the public interest to prosecute. They will not automatically assume that any witness is telling the truth but will consider all of those factors a) b) and c).
This is case involves identification. The CPS will look very closely at it for that reason. The CPS will look very closely at what evidence there is to show that this man, and only this man, could have committed the alleged offence and that it must have been this vehicle involved. Identification evidence, even when a witness claims to have seen the whole event and seen the suspect clearly, is always difficult and many a mistake has been genuinely made by witnesses. It's also a case of a sexual nature. Such cases used to require some corroboration of the complainant. Times have changed now, but the CPS will be looking closely at the circumstances of any such complaint, nonetheless.