I used to work in a solicitors in the claims department and dealt with a lot of motor insurance claims. Now I'm not legally minded but experience has taught me some things. Request a copy of the report taken by the policemen, you can also be a witness that your girlfriend was with you at the time of said incident. Also try and retrace your steps and look for viable CCTV footage of you with your girlfriend at the time of the incident providing evidence she wasn't in the car at the time. Were there any other people that you met on your shopping trip? Friends, work colleagues? If so then they too are viable witnesses. The last thing to do is to go to the location where the incident happened and locate any further CCTV which could show the vehicle that was involved at the time of the incident. I had cases where people asked us to check their car for damage and to prove that here was no work done, however they wouldn't allow us to do that as the insurance company said 'It could be a different car and there is no evidence of it being the same one.'
She does stand a good chance. However it's just trying to prove that she wasn't in the car at the time of the incident, it's not really about the car being involved. It's a bit like speed cameras. If you get a ticket saying you were speeding and caught on camera you can protest against it saying that you weren't in the car at the time and provide evidence that it can't of been possible. Hope this helps.