Also, as far as I'm aware, you won't be able to get back in using the same BP. Once the bar code has been scanned I understand it will not allow re-entry (I guess to stop people printing out multiple copies of the same BP). So you would need to get a new BP issued - not sure what complications may arise there.
Provided the system has been updated to reflect that you are no longer airside, then you have no problem re-entering on the same boarding pass.
If you want to leave, simply go to the central customer service desks near gate A9 and speak to the staff. They will log this information and then BAA agents escort passengers out of the building. They come by every half hour, on the hour and at half past.
If on a domestic flight, then the process varies only slightly in that we would have to call BAA to inform them that the person is on a domestic flight and then they would arrange for the passenger's photo to be de-logged from the system.