Unjustified and forced write-off of a car with just a bumper

If there was a zero rating I would give it with pride. Firstly the car insurance claims my car is a write-off after my car has been bumped at the back, with no mechanical problem, and now when I challenge the decision to write-off the car as the quote was over priced and to allow me to fix my car myself they are telling me stories and offer ridiculous buy back options which means my car is sold back to me as a write-off and claiming, this is from a manager and consultant that they will not mark my car as a code 2, which is total lies!!!!!I have provided the insurance with an alternative quote, which is way less as an all in repair settlement to fix the car myself, the quote was submitted since Friday the 9th June, till today the 13th they are forcing me to take a write-off as a settlement over a car with just a bumper at the back.