Assuming that your boot floor is still straight, and the gaps on the rear doors are as they were, then this is approx a 10 hour repair.. thats £250.00 at £25 per hour, £100 materials, and the price of a new bumper, plus VAT.. your going to be somewhere around £600 in total, thats allowing £200 for a new bumper (I don't know how much they are) that would put it beyond economical repair!
I would ask the insurance company, because of the type of car, and the problems replacing it for another one, if they could send out their engineer to view the car (speak to him personally) with the possibility of repairing it for a contract figure, as the damage is only cosmetic, and of course you locating a second hand bumper to keep costs down,. you may even be able to save the one you've got! just remove the bent steel backing, and replace with a straight one, after giving the plastic cover some heat to get it back into shape
Try and avoid the "buy the car back" route, you'll be buying back a write off!
90% of all the Insurance companies that I deal with, are very accommodating to this type of repair, ie, you should get somewhere with it! just be extra nice to the engineer, and the garage that the car would be inspected at!
Hope this helps!
Tom