A Cat B insurance write off cannot legally be repaired and cannot be put back onto the road in the UK. It must be broken up for spare parts. If you think it should not have been declared a Cat B then you could get Scotia Vehicle Inspections to inspect the wreck and write a report. If they say that they think it should be Cat B repairable, then you can use that to sue the insurer in the small claims track of the county court.

Alternatively if your car contains many valuable parts, you can demand to keep the car so that you can sell the parts or put the parts on another car. If any of the valuable parts have "gone missing" then you can sue your insurer for their value.