Well, if she's gone I'd report the breakdown of the marriage to the Home Office. Are you 100% sure that it is over? I'd want to be able to say yes to that before reporting it.
I'm familiar with US laws (worked for Orlando Police while in college). I know there you had to evict someone from your home and can't force them out. But obviously we are in a different country.
Why not change the locks while she's gone? She's affectively abandoned the property at this point, correct?
Yes, I would not allow her access to your bank accounts. You can remove her from your insurance. You could certainly call the police non-emergency number for advice (or CAB). I imagine that as she had permission to use the vehicle and take the car, there isn't much you can do. But again, when she leaves for the US, there is nothing stopping you from taking possession of the vehicle.
Tell her it's over, that you are (or have) changed the locks, that she can no longer use the vehicle, that you want a divorce, and that you've reported the breakdown of the marriage to the Home Office. Might want to wait until she's back in the US to tell her, just so she can't damage your property.
Do you think she really plans to return from the US? Do you guys have kids?