The lock in the boot of the MK2 on the factory central locking has a white plastic slider piece that is connected to the solenoid via an operating rod.
When you lock the car it slides it out of the way and the outside push button has nothing to push on so it doesn't unlock.
When you unlock the car the slider piece moves back into position and the lock button pushes onto the lock and opens it.
The above is also determined by the position of the lock barrel, as one is constant lock, one is central locking and the other is a fail safe to allow you to get into the boot.