That is the definition of an insurance write off, when repair cost are greater than the vehicle value,not just that damage is so severe it should not be back on the road, the car has been written off. You are absolutely correct that it can go back on the road (unlike cat A and B). But it will be recorded against your log book as a cat S (structural damage) write off. Personally in your situation I'd buy the vehicle back and use the cash to buy a new chassis/secondhand parts (good score on the sump BTW) to put back on the road but as I'd have no intention of selling the Land Rover the reduced value of having Cat S against the log book would not bother me, others may take a different stance dependent on whether they could have the vehicle off the road for a length of time, had the ability/space/time to do the work or intended to fix and sell.
I'll have to confess to not being fully conversant with the law but my understanding is once repair cost exceed vehicle value an insurance company (yours or theirs) liability is limited to the value of the vehicle. Whilst you can argue about the cost of repair and the value of the vehicle (worth getting an agreed value during insurance?) the premise that you can demand an insurance company to pay more for repairs than your vehicle is worth is new to me. For instance if the vehicle was a cat A or B write off it can never legally go back on the road if you exercised your right to have your vehicle repaired they would hand you a car back that you could not tax, MOT or insure.
