Your insurer will work out the market value of your car based on factors like the make and model, age, condition and mileage in the event of a total loss claim.
We can’t directly influence the total loss valuation, but if you believe the valuation is too low you should contact the insurer dealing with your claim and let them know.
Any request for an increase in total loss valuation will need to be backed up by evidence, so you will likely need examples (perhaps 4-5) of the same make and model of car (with the same specifications, age and approximate mileage) listed for a higher price than you’ve been offered. Autotrader and other trusted marketplaces could be good places to look for these examples. Bear in mind, this evidence won’t guarantee your valuation goes up.