Know what it is ?
I don't make any claims, I'm not a financial advisor, but this has worked for me a number of times, in fact I now buy cars with the intent of doing it.
If your car (or any other item) has been bought on Hire Purchase (not a loan), you usually have the right to return the goods (i.e. the vehicle) to the Hire Purchase company when you have made 50% of the payments due. This figure is usually stated on the front sheet of the Hire Purchase agreement.
You need to plan it in, and get the timing right with regard to minimise spending on road tax, servicing, tyres, and it may be even worth making a payment to reach that magic 50% payment level.
It's a good method of getting rid of a car that you bought on Hire Purchase when the car is worth less (selling privately or part exchange) than what you owe.
Most garage originated agreements are Hire Purchase (HP), including those with balloon payments etc.
The process is called a "Voluntary Termination", and - because it is legal, does not affect your credit rating.
The returned vehicle should usually be clean, roadworthy, the condition must be as appropriate for the age and mileage, V5 must be present, service record present, no accident damage etc etc.
I'm not a financial advisor at all, but I've done it a number of times.
What happens to the car ?
You phone them up (or write), pay a fee (usually £35 to £45), they come and inspect it, and take it away.