It was at the MINI Clubman VIP Preview event at Plant Oxford. In a presentation by a member of the Munich led design team, he explained that due to the position of the fuel filler (on the left side of the vehicle), it is impossible to postion a door there.
To get a door on the other side would mean moving the fuel filler rearwards, which puts it effectively in the middle of a crumple zone. Metal reinforcement would then be required to protect the fuel filler, which would in turn add more weight to the rear of the car, and affect handling.
Because of all this, the second rear door on the left hand side was ruled out.