Well they will usually write off if the repair value is uneconomical to repair, i.e represents more than I think its 70% of the value of the car which if you have it valued at £4,500 is £3,150 which is probably about right to put a quarter panel in a car and do all the work etc.
Its going to be shown as a category c car repaired anyway so why would you not just let them write it off, buy it back for scrap and get it fixed yourself anyway. As a non insurance repair you will probably get it done a bit cheaper. It makes little odds if you have already opted to go through insurance anyway.
You will benefit more financially if you just let them write it off and buy it back for scrap value