Sorry to hear, I know what you're going through, A twonk in a merc rear ended me a few months back ( at a round about too!) .
It WILL be classed a write off if you let them get their way. Apparently you can fight it, but I caved in and was persuaded to take the cash as mine was an easyish repair (still not finished it

).
Is the boot floor damaged? or is it 'only' the rear panel? I think my boot flooar was saved by the towbar & HUGE bumper iron that comes with the towbar.
How does it drive? If you don't mind driving around in a 'write off' (and prepared to have it taken through a SVA ) and can repair the damage, then you've scored.
I settled for £700, I bought the car for £750 a month before the twonk rear ended me, so I essentially paid £50 for the car, £20 for repair materials (sand paper and hammerite), £40 for SVA.
You'll need a receipt of sale to prove how much you paid for the car, they never asked to see this but asked if I had one.
You might need to find a few other cars of the SAME year & reg, VERY similar mileage, same colour, same 'extras', same valves, they never asked me for this but they 'searched' and found the average to be £700 for the GTD ( I also said I won't take any less ).
Remember it was NOT your fault, don't let them treat you as if it was your fault, don't let them push you around and most importantly be strong and forceful, but not abusive.
Also if it does get written off, make sure you can keep the car as part of the deal (even though it's yours they could still take it and scrap it! ).
oh and claim on
HER insurance not yours otherwise you'll have lost any no claims you have (unless you're protected).