Its done under the regulated agreement law of halfs & thrids.
Basically as an individual you are not supposed to be able to look after yourself and make an informed decision. They brought out halfs and thirds to protect us. If you have paid at least third of the invoice amount then they cannot recover the vehicle without going to court to reposses it if you get into money trouble. If you have paid 50% then you are quite within your rights to hand the vehicle back and walk away. The only wording on this is 'appropriate mileage and condition'. There are NEVER any mileage restrictions on an HP agreement, only lease agreements have them.
So you pay a deposit and you pay monthly payments till you get to 50%. 9 times out of ten the car is worth more than the remianing 50% unless its a mondeo/vectra type vehicle or its been purchased with a low deposit over too long a period. I refuse to sell vehicles on this basis as its basically negative equity the customer is trying to get away from.
Anyhow, you hand it back and as some have stated above you get a delivery driver collect it. Chances are you hear nothing further. Do you want to risk a bill though? You get a coompany that are switched on (They are all having to watch costs) and you could get a bill for treble the amount you expected. I would call a chips away type person and see if they can do anything.
Oh and if you think this doesnt count against you on finance records think again. I have seen records showing this and although it doesnt come up as a black mark, it is recorded so the finance companies in future will be able to take it into account.
Just for the record are we talking about a car you dont need anymore or one you cant afford? Why not just sell it, or is it in negative equity?