Read the T&C's on your finance agreement carefully. On a PCP agreement, the finance company decide on the GMFV (guaranteed minimum future value) before the start of the contract. They use the cars value, anticipated mileage & length of term to decide this.
You normally have 3 options at the end of the contract, 1) pay the GMFV and own the car outright, 2) get a trade in price and part exchange it if there's any equity, or 3) hand it back to the finance company. Option 3 should be your best bet, however, if you've exceeded the final anticipated mileage (that was set at the start of the agreement), you will have to pay a mileage charge, which can vary from 4 to 25p per mile, again look at your contract closely to see what you signed for.
Voluntary Termination may be an option, although only when 50% of total monies owed has been paid by yourself. If you're nearing the end of your contract and haven't missed any payments, you should be past that stage by now. Be careful though, any deposit big or small is gone, kept as part of the agreement. You hand the car back and walk away with nothing.
Above I did mention that there are normally 3 options at the end of a PCP agreement, however there are some Personal Loan PCP agreements on the go, these should be avoided as you don't get the option to VT. But worst of all, you Tend to have only 2 options at the end of the agreement, 1) pay the GMFV and 2) trade the car in, take equity or pay the negative equity. There is no hand back to finance company. These tend to be rarer agreements, hence read your contract carefully.
You could go down the route of complaining you've been mis-sold the finance, saying it wasn't properly explained at point of sale. Follow the complaints procedure on your agreement. A complaint getting to the FOS, results in a £850 case fee to the dealership, regardless whether you or they are in the right.
Good luck with this.