Then my experiences differ from yours. As I said, this is not off what I have heard, it's off what I have done. Maybe I have gotten lucky, maybe the rules aren't enforced, whatever. And as I said, someone else did it in June. Arrived on BA, transited upstairs (and they have always checked BP or ticket at the metal detector and xray) sat around in the concorde lounge (which I agree, shouldnt have happened for concorde lounge) until the flight to the US on American Airlines. They always let me into the Concorde Lounge, but they were clear that I was not to go to the MB spa for treatment. Shower was okay. Sad thing is that all I really wanted was a computer to check my email. Otherwise, I wouldnt bother and would just sit in T3 and watch the people go by.
All I can say is that I still disagree. I am most certainly not saying any of you are wrong, I am saying I disagree. Maybe its a theory vs. practice difference. But my experience has been 100% contrary to all of what has been said.