Even if a police officer didn't actually see you sitting in the driver's seat, it's quite likely that he'd be prepared to state that he did. If a court had to decide between the 'iffy' story put forward by you and your girlfriend and the sworn testimony of a police officer, who are they going to believe?
I'm sure that others will post here to lecture you about your behaviour. My response is based solely upon an analysis of the risks. If you plead guilty to drink-driving with no licence or insurance you might be sent to prison for, say, 3 months. (That would largely depend upon any previous convictions). On the other hand, if you try to cheat the system and get found out, both you and your girlfriend almost certainly will be sent to prison for about 2 years. (Conspiracy to pervert the course of justice is a very serious offence which nearly always results in a lengthy prison sentence).
If you really love your girlfriend, you shouldn't allow her to take the risk of going to prison for a long time because you haven't got the guts to plead guilty.
Chris
PS: I'm not actually convinced that your girlfriend was insured to drive your van anyway. Most, but not all, comprehensive policies allow the policy holder (i.e. your girlfriend's mother) to drive other vehicles (with only 3rd party cover) but this provision does not usually allow other 'named drivers' (such as your girlfriend) to drive other vehicles.