It is possible to leave T5 - there are a few previous posts about the procedure you have to go through to get back out. Not sure they would be keen to let you do this unless you had a really good reason.

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.

So in summary I would try to avoid this scenario!