Thank you everybody for your responses. Mazda UK responded and essentially indicated that they were disassociating themselves from the matter. Not covered by warranty, suggesting it was due to normal wear and use.
I have had three VW Golfs before to the Mazda and two Seats before that, and I have never seen any difficulties with brake rust within the first two years. Comparable yearly mileage as well. My current and former residences are not in proximity to the sea, and while I do not accumulate a significant number of yearly miles, I use my vehicle often and with much enthusiasm when road conditions allow.
I will take the vehicle for an extensive drive on many familiar winding routes and thoroughly test the brakes by progressively increasing the braking force to see the effects. Thus far, I have determined that executing forceful braking in a straight trajectory is steady and progressive, and the brakes seem to be as robust as they have ever been. It is possible that the corrosion may resolve; we will see.
I must express my disappointment with Mazda's answer, which I will include into my decision when selecting my next vehicle, likely next year.