The first thing you are told by your insurers is "You have to use our approved repairer". This is nonsense. Your absolute legal right is to choose your own repair centre. The choice is the customer's and no one else's.
The second thing you are told by your insurer, "should you choose your own repairer, you need to get two independent quotes." Again, utter nonsense, this is just another tactic to discourage you, by wasting your valuable time running around all the local accident repair centres to get two unnecessary quotes. You only need one quote, and we will take care of that for you by dealing directly with your insurer.
The third thing you will almost certainly be told by your insurers, is that "should you choose to use your own accident repair centre, you will not get a courtesy car, and work carried out will not be guaranteed to the standard of our approved repairer, and that the insurance company will not take responsibility for inferior work of an independent repairer". Clever but not true.