1. Milroy carries a wider range of parts compared to Prestige
2. Labor skills are equal or better at Milroy's
3. Certain parts are equal or cheaper at
4. Yes, at least has done for me.
Go visit his place and you could see for your self, the scale of operation he has, compared to the agents. Also, I doubt a typical BMW enthusiast would make a fuss if the car was maintained at Milroy's.