Sometime people dont know how to unlock a P6, I have seen clever people struggle. If you have owned one though it will be no problem.
It you can get a key to tun in the lock it should open, I have had one broken in the past.
In case it has been a long time since you last openned a boot. try putting the key in and just turn the key, and listen for a click, remove the key and push the button, you should feel resistance. You dont put the key in and turn the opposite way, listen for the click and push the button.
Common causes of not being able open in debris getting near the catch on the inside preventing it releasing. The catch also can slide especially if someone has had a go at it.
If you get resistance when pressing the button repeatedly press it in and out whilst pushing down on the number plate plinth, this can help bump it open.
If all fails and your boot floor and inner wings dont have a handy rust hole, you can get in via the battery box if it isnt secured or you can cut it out.
Worse case senario get another lid, they are pretty easy to get and cut the lock out.