Nothing wrong with that argument, most things like this are impossible to prove with solid evidence, but they are valid nonetheless. but inevitably, you'd have to go to London Tribunals for an unbiased decision. This would mean forgoing the discount.
If you do go there, it would be essential for your wife to attend the hearing so the adjudicator could ask questions. This would normally be by telephone. The civil law test of "on the balance of probabilities" is used, not the criminal test. Supporting your argument would be stopping there in the first place. Why would a normal motorist stop there if it wasn't something important ? As you say, nobody got in or out of the car. You could win this at the adjudicators, but you'd be in for £130 if you lost.