gap insurance covers you should the car be written off and your insurance company pays out less than it may have owing on it, hence bridging the gap.
So if you need to claim after a couple of years you'll only get up to £15k back.I was offered and took Gap Insurance by my HPC. Through Nissan Finance with Cardif Pinnacle. 3 Years, up to max of £15,000, single premium £299.
I agree 25K would have been a better, this was however against my usual position to turn down all additional insurance. 15K was all that was on offer and & £299 seemed a reasonable price, so I took the easy option rather than looking around.
At the end of the day you pays your money and takes your choice. We all hope we wont need it and that the premium will be wasted. Even if it does not completely bridge the gap it will go a long way particularily in the first couple of years and you will anyway be getting a new car to replace an old one.