Just as a point of order, it doesn't matter what an insurer "values" it at today. It's irrelevant.
Market value on the day it is written off is what matters, and that is easily evidenced by way of the vast volume of adverts for used vehicles. You'd be entitled not to it's trade value, but the the retail value of a similar replacement, and the ombudsman has become quite hot over this the last couple of years.