Few thing to consider when taking out 'shortfall insurance'.
There are 3 main sorts of cover.
Replace with new.
Return to Invoice.
Gap.
Replace with new will give you a brand new car, even if the list price increases.
Return to Invoice will give you the invoice price you paid back in full, if the car is 35 months old and written off and the insurer pays out £15000 but you paid £45000 the shortfall cover will pay you the £30,000 difference or up to the amount you have agreed.
Gap simply covers the gap between paying settling the finance owed.
So if your insurer pays you the book value of £15000 but you still owe £21000 they will pay the £6000 gap to make sure the finance is settled.
Now, gap is not really worth the paper it is written in, all finance houses these days will make sure that by the end of year one you are not upside down with payments, or it will be damned close, by year 3 you will owe less than the car is worth, that is pretty much certain at the moment due to the much lower guranteed final values.
And in year one where there is a chance of being upside down, 99% of insurers pay in full anyway, so no need for gap.
It is actually more useful on a used car.
Return to Invoice is the one you want, and be careful on price, I pay £4.80 a month to cover a £51000 730d for up to £30000 on return to invoice.
Nice thing about paying monthly is if I want out after 12 months I am not paying for shortfall cover that is not being used.
I don't like replacement cover, to be honest if I write a car off at 35 months I would rather something else, and no they won't give you the money as they mostly have deals with suppliers to get a model from stock at cheap prices. So Return To Invoice or don't bother.