Hi Keith....welcome to the Club
I can sense your passion for all things BMW from here!
Track Days are a great opportunity to test out your car's capabilities and it is a shame you don't have anything on your doorstep...the only ones coming to mind are Wiscombe Park and Gurston Down where they run the occasional Hill Climbs.... always a good day out!
I too am Advanced Driver and found it astounding when looking at Insurance quotes that this did not seem to impact the cost of the premium....far too underrated and something Insurers need to embrace to encourage better driving standards in my mind.
As for your point on low mileage cars....that is the actual issue, they don't get driven hard or far enough for the big and effortless engines to effectively clear out carbon deposits which then clog the injectors and turbo vanes.
Your Bimmers certainly seem to have good miles on them, which is also a sign that they have been driven properly (progressive driving to quote the IAM)
Good to have you onboard and look forward to reading your future posts.
Cheers, Trevor