GAP Insurance

Assalamu alaykum, Here in the UK car insurance is compulsory as we know. When buying or leasing a car on any sort of finance (including 0% interest) car dealers offer you an optional insurance called GAP insurance. This is because if the car is written off ( by accident, theft or fire) the insurance will pay out the current market value of the car not the purchase price (what you still owe). The insurance money goes to the owner ( the lender) and if there is a shortfall you the customer have to pay the lender..so the GAP insurance is supposed to cover the shortfall.. Is taking this GAP insurance permissable?