Have to say a similar incident happened when father clouted a road sign in the old TD... bumper was actually ok, bar a couple of snapped clips, but the rear crossmember was deformed, boot dented heavily (lights ok, though)
Basically...
Car insurance write off - why even bother ringing anyone? Waste of everyone's time so...
-New 2nd hand bootlid in the same colour off OOF
-Ratchet straps attached to something sturdy, like a tree, gatepost, another parked car etc.. pull crossmember and boot lip back out
-Fettle, mallets etc.. until boot shuts again.
-New 2nd hand bumper in similar(!) colour off OOF
Total cost about two days and £50ish... That's
no specialist tools (unless you count ratchet straps)
no specialist knowledge,
no specialists' time, just some common sense and a couple of days... well, that's if we discount the initial lack of common sense in having a car with worn wishbone bushes, incorrect camber at the rear, consequential greasy handling causing the car to 180 on a country road in the first place, of course