I just read through that section in the manual. I suspect that "maganoo" is right: a firm pull on the lever inside and a bold push should break them free.
The manual also lists a method if the boot won't open because of a dead battery (pg. 30):
"Tailgate
Manual release in the MINI Convertible
In the event of an electrical malfunction, you can also unlock the tailgate manually.
1. Unlock the catch for the rear backrests with a master key or ignition key [top right of RR seat, facing rear]
2. Fold the rear bench seat upward and remove the hook attached to it
3. Using the hook, reach backward between the convertible top and the rear backrest to pull one of the levers for releasing the folding rear backrest, refer to page 86
4. Fold rear backrests forward
5. Pull the handle, arrow. The tailgate is released."
This just provides a mechanism for pulling the same lever that you get to from opening the tailgate.
JR