You have two options:
1) The Mini retrofit armrest. This is designed to be added after the car is built and has two metal legs that go down to the floor, either side of the console - shown dotted below:
2) Someone here has fitted the original armrest that sprouts out of the rear cupholder area:
But as well as the armrest, you need to fit:
- new plastic parts for the cupholder (items 17 and 18 below);
- a new support frame (item 3 replacing your current item 2) under most of the centre console;
so this is not a 10 minute job and if you want a dealer to do it, it will cost you.
You can use
RealOEM.com to work out the part numbers you need and then put those part numbers into a Mini dealer's pricing website (I use
Cooper Mini) to find out how much the parts will cost you, if you want to do the swap yourself.