If the car is to be scrapped, then I believe you can claim paid but unused road tax.
Note however that scrapping the car, and the car being a write-off are not necessarily the same thing. A car can be a financial write off in that it is not worth it for the insurance company to pay the bill quoted for the repair as it exceeds the value of the car. However, a car written off in such a way could still make it back onto the road because Joe mechanic might buy the scrap and put it back on the road on the cheap. In the latter circumstance, I expect you would not be able to reclaim unused road tax.