You can dispute the insurance company's decision to write the car off and insist on a repair, but ultimately they may decide that it is economically unviable due to the cost of the repairs exceeding their set percentage of the vehicle's worth. However you may also wish to challenge their valuation, which is where evidence of its condition and service history will be useful. Even if this is unsuccessful, you can request to buy back the car as salvage once you have accepted a payout and pair for the repairs yourself. The vehicle will still have a category marker attached to it, but if you are keen to keep the car this is the best outcome.