If you cannot nominate another driver then the only way to contest it is to be prepared to go before the court, tell your story and hope that the magistrate believes you and is sympathetic enough to accept that it was reasonable in your situation not to be able to nominate the driver in question.
The requirement to nominate someone is a strict requirement normally done by statutory declaration. To falsify a statutory declaration is a serious criminal offense punishable by a term of imprionsment. Accordingly, you must be very careful not to nominate someone unless you have good grounds for believing that person was the driver, and based on what you have said, it appears you do not have such clear grounds.
Whilst it is a matter for you whether you contest the charge, unless you can simply persuade the police to withdraw the infringment notice, you will need to contest the matter in court. No doubt contesting a charge on the basis of confusion over who was at the wheel has succeeded many times, however, it will simply depend on whether the court believes your uncertainty as to who was at the wheel is both genuine and reasonable, and it is not possible to predict how a court will decide those issues before you and/or your friend give evidence on the matter.
If you need assistance considering what is involved in defending the matter before the court, the following links may assist:
I appreciate that may not be the answer you were hoping for, but I trust it assists.
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