Unfortunately if the cost of the repair is more than 60% of the value of the car, they'll right it off. So repairs over a grand means the car would need to be worth more than £1700 for them to do it.
You can agree to buy the salvage from them (they'll want 35 - 40% of the book value for it) and then repair it yourself. However your car will likely have a Category D or possibly C recorded against it in the insurance register which means future resale value will be 60 - 70% of a normal car.
In summary, a lot of hassle. So I'd go for a decent settlement - argue, argue and argue with the insurance co. They will keep arguing back but will back down, eventually. Get some samples off Autotrader of same age/mileage/condition cars as proof of the cars value. It may take a couple of weeks of daily argumentative phone calls but they will settle for a decent value eventually. You have to remember the whole process isn't there to make your life easy - it is there to make the insurance company shareholders money. Hence they will try to get away with paying you the least possible by making the process frustrating so that people give up and accept whatever has been offered.