Don't want to be stating the obvious here, but if they write it off then they will make an insulting offer to you. I had a *mint* sharknose 5 series auto for the missus which some silly woman drove into, and despite there being an IDENTICAL car available for sale at the time (only a couple of months back) at 2k, they offered me £400 and said it was not a classic car, despite being on a classic policy. This one was, I know there are some sheds about but it was a rare model and colour and like I said in perfect nick.
First thing to do is collect evidence of current/recent examples of the same spec, or as near as you can get it, then the next step is to bombard the assessor with them via e-mail until they get sick of you and their manager is pressing them to get the call closed off. In the end I got a grand, which was a fair price in the end as my other half had admittedly already scraped it up the side.
Remember, if it is not your fault (and especially if they have admitted liability) then you do have the option of rejecting their offer and suing the driver for the damages instead. All that happens is that their liability is covered by the insurance company anyway, and so they have to pay up just the same but with the added court cost and hassle of their customer being annoyed. It's not that you'd actually want to do this, but it's a good tool to threaten them with - for example, I was repeatedly told that it was impossible to pay out more than £800 for the BMW for all sorts of reasons, but when I said 'OK then, no problem, thanks for your help but I'll take the driver to court instead' then mysteriously another few hundred quid appeared the next day!
Please note, I'm not saying 'defraud the insurance company', nor did I do that - but we all know that things like timing belt changes, etc are expensive and aren't taken into account by their computer systems so this is a way to ensure that you get the correct value of your car back when something has happened that is not your fault. Perhaps they will make you a reasonable offer anyway, but we all know that doesn't happen every time.
Good luck with it!