Cah in lieu (CIL) settlements are normally made at the insurers discretion - you'd be surprised how the amount of requests for this type of settlement increase around xmas!
We normally go down this route on cars that have minor damage, but are technically total losses - we're talking paint scratched, but car needs re-spraying, so no reason to take the car off the road.
Bedknobs is spot on with the reason we don't like to go down this route, but at the end of the day, so long as you don;t come back and try to say you've had another accident with exactly the same damage, and you can prove that you've repaired your car since getting the CIL, we don;t particularly care.
CIL kind of absolves us in a way - if a policyholder insists that they want a CIL as they can repair it cheaper than we say, then we may accept this. The condition of a CIL settlement though is that you can't come back and then say "I've found more damage and want more money".
As a general rule, I would say most people who go down this route are trying to make a bent claim