Check your finance agreement, assuming it's regular automotive insurance which should be secured on the car (e.g. Renault Financial Services) there will be a 'termination' section in there, if you've paid more than 50% of the car off you should be entitled to hand the car back and walk away.
Keep in mind that this is 50% of the 'total amount payable' i.e. if it's a PCP, it's the full value of what you borrowed including the final 'balloon' payment and you may be quite short of what you need. If it's straight HP and you had a decent part exchange car or a reasonable deposit it's easier generally to work the figures out and you are normally close to the (50%) figure you need sooner.
It can be quite handy especially if you are in a straight HP agreement and facing fairly catastrophic depreciation, although of course finance companies don't really make a big effort to tell you about this clause...