I am wondering if you were able to have this issue fixed. I have a MINI 2015 and experience exactly the same problem.
I have my MINI for 7 months. Summer and fall have passed and winter is almost over, but in every season the interior of my MINI always gets overheated as soon as I drive more than 1 hour. If I stop driving after 1 hour and leave the engine off for 30 minutes, the interior can get to 100 degrees depending on the outside temperature. Sometimes it is 40 degrees outside and 70 degrees inside without having turned on the heater system. I have already taken the MINI for inspection, but I was told that nothing is wrong with the air conditioning system, nor with the heater system. In addition, I was told that is the way the MINI is designed. I would immensely appreciate if many people would respond to my post because I don't know what to think or to do. Please note that I am a type of person who is always cold, but not inside of the MINI. I leave in California and rarely need to turn on the heater system inside of the MINI. To the contrary, while I had an old Honda, I often had the heater system on high. Now with the MINI, often I have the air conditioning on during very cold days.