Originally Posted by
cme17
Yes - MAD. Flying LHR-LPA (via Mad) with BA and then IB you need to do security at MAD.
But not the other way around.
Taking the train from T4 to T4S you stay airside.
I believe that the reason is that arriving at T4S you are mingled with passengers arriving
from outside UK/EU/EEA who might not have been screened to EU standards.