They rules say they have to pay the market value - what it would cost to purchase an equivalent car.

Negotiate hard, do not take the first offer, and don't come to an agreement until you are happy with the amount.

It helps to gather evidence to support your demands. Find adverts for similar cars on AutoTrader, etc. You can also cite the condition being immaculate, the history being known, the servicing being meticulous, etc - and use this to argue up from what similar cars are being advertised from. Anything you can show to support what you think your car was worth before the accident, show it. You can certainly argue that you need to be compensated for the 'uncertainty' involved in purchasing another similar car.

Above all, remember THEY DO NOT GET TO DECIDE THE PAYOUT. They need your agreement.

As an alternative, if you know a reasonably-priced repair garage then try and get a private quote to fix it for cash in hand with no insurance involvement. You might find that the amount is lower than the write-off threshold (the repairers estimate will always be much higher when the insurers request it, for a variety of reasons that I won't go into). Then you can ask the insurers for cash in lieu of repairs and just pay to have it fixed.

You will probably be offered far short of what you consider as the value. There are various options inclding buying the car back in but these are fraught with difficulties as if not done in a certain way it will be listed as a category D write off and hence worth much less if you sell it.

If you want to give me full details of the collision, your contact details and those of the other driver I will be happy to advise further.