The at-fault insurers' liability lies up to the market value. Based on what you have said, it will always end up being uneconomical to repair as the repair cost will likely exceed the market value. If they were close, you could ask for a cash in lieu settlement, or agree to use second-hand or non-original parts to reduce the repair cost, but it does sound even with this the car will be a total loss. You could agree to retain salvage.