providing the agreement is a regulated one by the FSA you have a number of options:
halves:
providing you have paid off a minimum of half of the 'total' amount payable and the vehicle is is good condition for its age including wear and tear you can hand the vehicle back to the creditor. This may be shown on your credit file as a VT (voluntary Termination) this is different from a settled agreement
thirds:
providing you have paid a minumum of a third of the total amount payable, if you default on paymnets the asset cannot be claimed by the creditor without a court order
If you have a PCP (not lease purchase) where the car has a MGFV (minimum guaranteed future value) when you get to the end of the agreement if the vehicle is not worth what the GMFv is you can hand the vehicle back irrespective of the equity situation OR you can px and use any positive equity towards your next purchase OR finally you can pay or refinance the MGFV and own the car outright
Ownership of the vehicle under any HP/PCP/LP does not transfer to the registered keeper until all payments are made including any option to purchase fee's
You can also sell the vehicle. please remember that you will need to pay off the amount owing to the finance comapny or the car will still show on the HPI system
HTH
Kenny