Don't accept their first offer: do some research about what a similar car to yours would cost (get lots of examples) and be assertive. When DH wrote my mum's car off the insurance company initially offered abou £2000 less than the replacement. It was time consuming but I got them to pay the true price in the end.
Just because the cars been written off it doesnt mean it has to be . If the costs of repairs are less than the amount offered by the insurance or you want to pay the extra you usually can .the car will be classed as a category D ( I think) write off and you have to have the repairs checked at a DVLA place but the car is then insurability and you can keep it. We did this with one of our cars once .
some tw*t ran into the back of my (tow) car on wed, the insurance company have written my vehicle off they have offered me £1900!! it is virtually impossible to replace what i had for this should i a) buy a small banger and accept i can't tow for a bit b) buy a 25 year old landy extra info required for decision i drive 30,000 miles per year, extra fuel costs would be significant in all honesty i rarely compete between now and march (though was intending to go to weekly rc training i haven't in previous years) in may next year i will get policy money which i had always intended to buy new tow car with so could buy one then if not before reliability is essential as my work would be very unsympathetic if late/absent over transport issues