They exercised gross negligence that nearly resulted in the destruction of your car and could easily have resulted in deaths had they hit pedestrians or another vehicle. They chose to road test a vehicle at an inappropriate speed without first static testing or low speed testing the critical system that had been worked on, and didn't even apply the hand brake when things went wrong. The service manager abdicated responsibility by allowing a new and inexperienced mechanic conduct a critical safety check.
I'd want the vehicle back in exactly the same condition it was handed over to them, with the repair, all for free - they have, after all, reduced the resale of your vehicle because of the accident, even if all the damage is made good.
It was entirely avoidable and was down to their negligence. Their insurance will pick up the bill, not the workshop, but if their premiums go up as a result, then it may at least serve them a lesson to be more diligent in the future. It certainly shouldn't be of concern to you. Frankly, on top of the full repair, I'd have taken the courtesy car or expected compensation for the inconvenience of the duration of the car's repairs and for the loss of the holiday. SOme people are far too comfortable in mistreating other peoples' property.