@Nigel *******
, there is no "grace" period of 7 days unless you are currently working for BOI/IEAT company. you go to the labor office to cancel your WP, and bring the WP cancellation receipt to the immigration office to cancel your extension. Many people do it in one day. Also, you can apply these cancellations several days in advance. How long in advance depends on your local offices, though.
You can try to apply for another extension which will be denied so they MIGHT give you up to a 7-day-to-leave stamp. You want to apply for this extension on the very last day because 7 days to leave the kingdom would start on the day of application, not to be added if it's granted.