You will get a pro rata refund of the advance payment made for the written off car
You wont get any money back for extras you may have purchased when you got the written off car
They will try and get you to take a hire car on an arrangement where the third parties insurance pays the bill, its quite a weird arrangement where in fact you have to sign a form where you agree to pay hire charges if the other parties insurance do not.
Once written off you can simply order a new car, but have to pay the advance payment (hopefully using whatever pro rata refund you received to put towards the advance payment)