You are talking about modifying the braking system.
There are plenty of people working on their own cars doing basic maintenence and the like and always will be good on them, I am one. There are also plenty of superb back yard mechanics aswell as some shonks.
You mentioned someone from an insurer recommending drilling holes in backing plates and adding breather tubes, This is a modification to the braking system and illegal for yourself or a competent back yarder to do.
By all means do what you wish to your car

its yours just be aware.
Have an accident where your car in inspected by the police, they will ask you who modified the braking system. If yourself definately a fine and possible jail depending on the result of the accident, any mechanical work done by a backyarder which you paid for means back yarder will under current laws in NSW do jail time. Ignorence of the law is no excuse either. We are talking modifying the braking system not a tune up here.
Before embarking on any mechanical work ask around get qualified advice and if using a backyard mechanic get plenty of testimonials, there are plenty of people on this forum currently building cars or parts there of who have learnt costly lessons. Check on other mini forums for other opinons aswell there are plenty out there.
Cars are designed by engineers and developed with years of industry knowledge, even if mini's hydrolic brakes are simple and easy to work on they are the way they are for a reason.
If someone at Shannons is telling you what you tell us he did, he shouldn't have a job. Get his name?
Steve