Anyone has recent experience using T3 CX/QF lounge on a BA T5-to-BA T5 long transit? Checking for a friend who is interested in doing that. Is my understanding right that he can attempt to do this by pretending to be transferring to a T3 flight after arrival on T5, take the bus and clear T3 security (not clearing ay T5 security), tell the security person he wants to use the CX/QF lounge if questioned, then enter T3? What I am not sure is, on his way back to T5, how would he exit T3 to go to T5? Would he have to clear UK immigration somehow? I recall arrival and departure are segregated at LHR, right?

Thanks in advance!