Gutted
I know how much you love that car and I was looking forward to making it perfect for you.
That is most definitely an insurance write-off. They won't even contemplate trying to repair it with that sort of metalwork damage. That's way beyond Dean the dent guy's talents too - the metal is torn, and the weld seam to the lower rear panel has split.
The only way to repair that would be to cut the corner off an undamaged shell and weld it in. That would take some serious welding talent, but its possible - I know a guy, obviously. You would then need to re-paint that quarter panel, blending up the rear pillar somewhere. FWIW, 75% of the D2s I see have had the rear quarters painted at some point. Bolt-on parts could all come from Prestige. Mike and I are about to dismantle a silver '02 4.2qs parts car so the corner could come from that, possibly also other parts which Prestige don't have (I think they do have silver-blue panels).
Re-shelling is an option (in fact I have an FE shell for this exact scenario should it happen to an S8), but you would lose your car's identity in favour of the new shell. You would really need a 3.7 shell to transfer all your running gear in to, as otherwise you'd end up with a modified car (a 4.2 with a 3.7 engine for example) which is messy with insurance. You'd either need to find a donor with acceptable bodywork (expensive), or paint a scruffy one. My rescued-from-certain-death W-reg 3.7 would fit the latter category, and I'm not particularly attached to it yet so I'm open to discussions there.
If you are suitably attached to it to repair it then I would suggest trying to negotiate the best cash in-lieu of repairs settlement you can from the insurance and use that to get it fixed yourself. Otherwise, get the best payout you can for them to write it off, but make sure you get to keep it. Its going to cost several thousand to fix it to an acceptable standard so you're going to end up out of pocket one way or the other
Tell your insurance asap, whether you have the police report yet or not.
Happy to help however I can obviously.