Roux v Salvador (1836) 3 Bing NC 266 per Lord Abinger CB at p.286:
The underwriter engages, that the object of the insurance shall arrive in safety at its destined termination. If, in the progress of the voyage, it becomes totally destroyed or annihilated, or, if it is placed, by reason of the perils against which he insures, in such a position, that it is wholly out of the power of the assured or of the underwriter to procure its arrival, he is bound by the very letter of his contract to pay the sum insured. But there are intermediate cases – there may be a capture, which though prima facie a loss, may be followed by a recapture, which would revest the property in the assured. There may be a forcible detention which may speedily terminate, or may last so long as to end in the impossibility of bringing the ship or the goods to their destination. There may be some other peril which renders the ship unnavigable, without any reasonable hope of repair, or by which the goods are partly lost, or so damaged, that they are not worth the expense of bringing them, or what remains of them to their destination.
In all these or any similar cases, if a prudent man not insured, would decline any further expense in prosecuting an adventure, the termination of which will probably never be successfully accomplished, a party insured may, for his own benefit, as well as that of the underwriter, treat the case as one of total loss, and demand the full sum insured. But if he elects to do this, as the thing insured, or a portion of it still exists, and is vested in him, the very principle of the indemnity requires that he should make a cession of all his right to the recovery of it, and that too, within a reasonable time after he receives the intelligence of the accident, that the underwriter may be entitled to all the benefit of what may still be of any value; and that he may, if he pleases, take measures, at his own cost, for realising or increasing that value. In all these cases, not only the thing assured or part of it is supposed to exist in specie, but there is a possibility, however remote, of its arriving at its destination, or at least of its value being in some way affected by the measures that may be adopted for the recovery or preservation of it. If the assured prefers the chance of any advantage that may result to him beyond the value insured, he is at liberty to do so; but then he must abide the risk of the arrival of the thing assured in such a state as to entitle him to no more than a partial loss. If, in the event, the loss should become absolute, the underwriter is not the less liable upon his contract, because the assured has used his own exertions to preserve the thing assured, or has postponed his claim till that event of a total loss has become certain which was uncertain before.
Moss and Others v Smith and Another  EngR 155; (1850) 9 CB 94; 137 E.R. 827 per Mr. Justice Maule at pp.102-103:
If the ship is actually lost by a peril of the sea, or any other peril covered by the policy, the assured may call it a total loss. It may be that the injury sustained by the ship is irreparable with reference to the place where she is; for instance, the ship may have met with the disaster at a place where no workmen of requisite powers are to be met with, or where the necessary materials are not to be found, so that to repair her there is altogether impracticable: and in such a case the loss would also be a total loss. But, short of that, it may be that it may be physically possible to repair the ship, but at an enormous cost: and there also the loss would be total; for, in matters of business, a thing is said to be impossible when it is not practicable; and a thing is impracticable when it can only be done at and excessive or unreasonable cost. A man may be said to have lost a shilling, when he has dropped it into deep water; though it might be possible, by some very expensive contrivance, to recover it. So, if a ship sustains such extensive damage, that it would not be reasonably practicable to repair her, seeing that the expense of repairs would be such that no man of common sense would incur the outlay, - the ship is said to be totally lost.
… The ordinary measure of prudence which the courts have adopted, is this, - if the ship, when repaired, will not be worth the sum which it would be necessary to expend upon her, the repairs are, practically speaking, impossible, and it is a case of total loss.