Being an approved repairer is a license to print money. As soon as an insurance job comes in to these guys they charge like the light brigade. I can't just blame them though, you would think insurance companies would make sure they were getting value for money.
What I don't understand about this story is why the person that hit your car didn't just offer to pay for the repair. Surely they must be paying a similar excess to everyone else, around £400-£500. If I scuffed a bumper that's what i'd do. Insurance claims are a last resort.