Yes you will still owe the Finance company the difference. How big would the difference be if you sold? Maybe you could get a loan to pay off the Finance company which at least would get them off your back if you wanted to clear that loan.
What interest rate are you paying on the car finance?
Isn't there something odd about HP agreements? If you haven't made half the payments, the bank can repossess it. But I don't know if it works the other way around. If you haven't made half the repayments, can you just hand back the keys? I doubt it, but worth checking out.
You could also set up a meeting with them and explain your position.
First question - is this a car loan or hire purchase agreement?
if it's a hire purchase agreement and you have paid more than half the hire purchase price (this will be on your agreement) you can return the car and owe nothing (as long as it needs no repairs). This is called retunring the car undre the half rule.
If it's a hire purchase agreement and you haven't paid more than half the hire purchase price you have 2 options:
1. Pay off more money to bring you up to half the hp price - if you are close to this, then that's worth considering.
2. You can surrender the car. The car finance company then sell it (for very little) and you owe them the outstanding amount. E.g. if there's 20k outstanding on agreement & they sell car for 8k, you still owe them 12k.