Stu Dent's advice is not quite accurate on the finance company's right to take possession. If it is a hire-purchase or conditional-sale agreement, and you have paid at least one third of the total amount payable, the finance company must obtain a court order before they can take possession, but if you have stopped making payments and have made no reasonable payment offer, they would have little difficulty in doing this.
If you have paid over half of the total amount payable, you can return the car to the finance company, and you will not be liable for any further payments (unless you have failed to take reasonable care of it).
If you still have a copy of the agreement, it will state on there the exact figures under sections headed "Repossession: Your Rights" and "Termination: Your Rights".