You have a CIVIL liability to refund the money to the insurance company. (i.e. they can make a claim through the civil courts if they don't get their money).
However if you wilfully withhold the money (rather than just asking for time to pay it back because you're skint), you'll be committing a CRIMINAL offence under the Theft Act 1968, which could lead to a fine (or, theoretically at least, a prison sentence) and a criminal record. (i.e. you've not 'stolen' the money yet but you will have done if you deliberately fail to return it).
Chris