The question turns upon the issue of ownership. Who CURRENTLY owns the car? You say that you do. Are you sure? When does property pass? That's the real issue at stake here.
The TERMS and CONDITIONS of the CONTRACT should show when ownership should pass. Being the registered keeper is distinct from being an owner. You can be the registered keeper of a million cars and own none of them. ROLLO is on the right lines in his advice (above). Sure he/We believe you, but can you prove it?
Being a registered keeper MAY give you a presumption of ownership but that presumption may be very easily rebutted by the T & C of the contract. So that's your starting point.
If the contract shows that you had to do X before aquiring ownership, and you failed to meet that contractual obligation, then reporting the car as stolen may (though it's unlikely) give rise to a charge of wasting police time. If you go and drive it away you then you could be charged with theft. So be very careful.
I am not doubting your integrity. I am just warning you that being the keeper does not make you the owner, and that you may have no legal right to the property, however much this may be at odds with your position morally. If you are not the owner and you take possession of that car without the owners permission then it is you who commits theft.
If you are in the right I suggest that your remedy lies in a 'Breach of Contract' action - supposing you can prove the breach. Do not drive away the vehicle!! Being morally right doesn't make you legally right. So don't get mad, get even!