Now that I've been able to sleep on all that's been mentioned, I think the best thing to do if they do write it off is simply buy it back, then fix it up and pocket the change left over.
The car is at BMW, so it's in legit hands at least, and sure it will be a cat c if it comes to that, but as I said, it's a car I'd happily keep for the remainder of its life, so resale value wouldn't be a bother. I suppose if I did sell it one day, then if whoever buys it is like myself, then this might not be an issue either once they see the circumstances paperwork and condition of the car anyway.
So no matter what happens, my car will be back on the road.
And may as well pay the bit extra for the whole car to have a fresh lick of main dealer paint with that in mind then.