I feel sure you will recieve some different answers to BOAC's. In my opinion it is NOT part of normal duty period, it is there to cover unforseen circumstances. I, and a number of others, have refused to use it (all had valid reasons) and have never had any "major" complaints or career threats from the company.
For Slim.
You CAN depart home base knowing you will go well into discretion. You can also pick up an additional period to get you home.