Ask for the photos and if they are inconclusive give them a big list of mates who were all 'sampling' your car at the time as you decided that it might be up for sale (obviously make sure they have proper insurance). Get witness statements from them all and say none of them rememember being flashed.

You are only legally obliged to tell them as best as you can who was driving at the time of the offence. I'm fairly sure you cannot be prosecuted any further if you do this to the best of your knowledge.