If you want to sell the car, then you need to make the buyer aware that the car is on finance.
Usually, the buyer will then arrange to pay the finance company the amount owed and yourself any extra amount (so if the car is sold for �14,000, the purchased will pay the �13,100 to the finance comapny and �900 to you).
If the car is not worth �13,100, (lets say its worth �12,000) then the buyer should ask for you to pay the difference in finance (�1,100) and then pay the finance company the �12,000.