I arrived a few weeks ago to T5 and wanted to go to T5 to meet someone in the lounge but I was departing T3 and only had a T3 BP I could not get through the official channels at T5A Flight Connections they just would not let me though with a T3 BP in spite of me have 5hrs transit time.
I have to say that in spite of great efforts by c-w-s and others to explain this whole issue of switching terminals for lounge access I am still totally confused as to what is allowed, what isn't... what is possible and how and what isn't. Sorry. I also have the feeling it also depends on getting compliant staff to actually achieve it.
I have decided that for now I will stick to the lounge in the terminal for which I have a BP!