Not all companies will allow you to buy the salvage but if they do tell them you want it and they simply deduct the salvage value from the final settlement figure
most insurance companies do let you keep the salvage now as it saves them a lot of money having to dispose of all the wrecks,basically it allows you to put it back on the road with the money they pay you out with,saving them even more money on other costs.
As mentioned previously, you will get an enginees report stating the value of your car and the salvage value of the car too. If you buy it back they will deduct the salvage value from your final settlement figure. Just be careful as to which catergory it has been written off in, as with certain catergories once you have repaired the vehicle DVLA may want to inspet the vehilce.