C'acas. It is not a NO NO at all! How do you use the lavatories?! Do you claim you never go and have a chat with the CC in the forward or rear galley?!
You should mate!
Because in the airlines I've flown for since 9/11 it is an absolute NO NO in the company orders. There were special security procedures in place for pee-time. Even if there was a pax disturbance in the cabin the pilots were forbidden to leave the cockpit. I'm not saying I agree or disagree with said orders, only that they were in force. Your company has different philosophies. However, I thought this NO NO order was EU wide. Perhaps you are somewhere else.