Gutted mate! I would call the police again and have them initially contact the garage for A) taking the car with no consent and B) not reporting the crash to their insurance. I would also contact VOSA and have them investigate.
Seems he's talking pish, the clutch kicked out and power stuck? They shouldn't have been driving and secondly I'm sure VOSA would inspect the car and confirm there was no mechanical issue.
100 percent seek legal advice to recover the car costs, your out of pocket costs for not having the car and then the fact they drove without permission.
Best of luck with this - could have been resolved easily and he decided not to. We all know how quickly a thread or video can go viral and sink a business!!!!