If you cancel in the middle, strictly speaking you are breaking your agreement to insure. These costs have to be paid no matter how long the policy runs so often the amount of any refund is quite small. The best advice is shop around carefully before you buy - then stick with it for the full 12 months. I paid hundreds of pounds more for my car than the insurance company is offering now it has been written off. The insurance company will pay you the value of your car at the time of the accident.
All cars deteriorate from the moment they leave the showroom.
If you are unhappy with the value being offered for your car, after it has been written off, send the insurers any proof you have - like adverts for similar vehicles or the appropriate pages of price guides. If you think it's worth it, you could get an independent engineer to look at the wreckage and give you an estimate of its pre-accident value. It's higher, so if I had a fire I'd just move to another house. That's easy to say but not always as easy to do. It is extremely rare to find a house is burned to the ground.