It doesn't have to be subject of an insurance claim to be put on the register.

I'm an engineer for a fleet company and I can write a car off and put it on the register if I don't think it's worth repairing for any reason what so ever.

My biggest client is insured TP, and if for instance the cambelt snaps on one of there vehicles and they owe 2k on the lease and repairs are 3k I'll settle the lease and sell the vehicle to our salvage company who catagorise it and put it on the register. No insurance companies involved what so ever.