Any vehicle that has rear disk brakes including the Mini has to be careful. If the handbrake is applied when the disk's are warm/hot and they cool down, then the disks contract and the brake will partially release.
To overcome this potential problem you should do the following: -
a) Before applying the handbrake press the footbrake hard down, then with your foot still on the pedal apply the handbrake.
b) Engage 1st gear or reverse depending on which way the car is facing.
c) Turn the steering wheel angle so that the front wheels will touch the curb if it rolls back or forward.