surely the police CAN do something, without having a pop, you haven't explained it that well, so if you relayed the same to the police they wont act.

But, if Iam understanding it, they attempted to jump into the car and drive off but you got in before they could - what did they do at that point?

Basically you should be reporting an attempted car jacking, and should have taken a note of their registration.

as has been said, these cars WILL attract dodgy characters, so when selling you need to make sure you ass is covered and that they don't get to drive the car until the deal is done, then they can have a drive once you have seen the insurance paperwork or have their money.