It's not really a grey area at all. If you don't know who was driving you must convince the court that you have taken all reasonable steps to find out. If you don't do that you will be convicted.
There have been one or two notable successes with the defence against "failing to provide driver's details" charges. Here's one:
NEIL AND CHRISTINE BUCK SPEEDING CHARGE
It's a little misleading because it's headed: "Neil and Christine buck speeding charge" when in fact it was only Mrs Hamilton in court and she was not facing a speeding allegation but of failing to provide driver's details."
That said, you need to be careful:
A MOTORIST has been convicted of speeding - even though he told magistrates he should be cleared because Christine and Neil Hamilton had been.
You need to convince the court that you did not know who was driving and, having exercised "reasonable diligence" you have been unable to establish who was. You haven't mentioned anything, so what have you done up to now to try to discover which of you was driving? One thing is absolutely certain: simply turning up in court to say "dunno who was driving" will not secure your acquittal. All the press reports seem to indicate that is precisely what Mrs Hamilton did. But I have my doubts.