My wife wrote off our 2008 E92 320i a couple of years back.

My learnings:

- don't accept the first offer
- decide what you will accept within reason, don't be greedy
- hopefully the car is not under insured
- do your research on similar cars on any car website you can
- show any differences in mileage, spec, condition, price with regard to your car
- use the revenue's OMSP as 1 piece of data
- check with a few dealers what is realistic to expect
- see if same dealers would be willing to put something in writing as to the value (they might if they are getting business from you later)
- compile all of your data into a spreadsheet or similar to help your case and send it to them

In my case the fact the car was a BMW, with extras, good condition,etc... I felt the claims settlement person I was dealing with understood and it was not the average joe car everyone has. When I made may case we managed to secure a few thousand extra and to be honest I felt I could have pushed them even a little more, but in the end we needed to get another car sorted and the offer we had was good enough to allow us do that so we took it.

Best of luck getting sorted.