I have put two different rear-facing infant car seats in my 2011 Cooper S on several occassions.

Some thoughts on getting the seat installed: The newer latch system that car seats use is nearly impossible to clip in due to the bucket shape of the rear seats, which also makes the child sit more upright and looks quite uncomfortable. The seat's movement from right to left was far too much and made me quite uncomfortable. Car seats are not supposed to move like that when properly installed. The tether off the top of the car seat has to clip below the front seat, which slides back and forth, so once you get the seat locked and latched, nothing can be moved ever or you have to readjust everything. This whole arrangement makes for an uncomfortable infant and an uncomfortable front passenger. 

Now lets discuss getting the seat in and out of the car. You can't do it with any degree of ease. I had to move the drivers seat forward, squeeze the carseat in through the triangle created by the front seat/seatbelt/frame, then while holding the carseat inside, climb in after it and sit in the back seat behind the driver to have enough reach and control to lock the seat into the base. When you change to a rear-facing seat without a base, doing this with a heavier, wriggling infant is not any easier.

Doing this occassionally when I watch my niece is a huge pain in the butt, even in good weather. I don't know how this experience would compare to using a carseat in a clubman or a countryman, but I don't think I could EVER load and unload an infant from my MINI everyday especially in the winter. Yuck.

I hate to admit to a MINI's shortcomings, but it is NOT a carseat friendly vehicle. Be warned.