Originally Posted by lhrpete

UK law prevents a passenger from entering a terminal that they do not hold a boarding pass for a departing flight from.

Originally Posted by KARFA

yes it’s perfectly doable at T3. Follow the signs for flight connections T3. When you get to the bp check before connections security explain you want to use the amex lounge and they will let you past… It’s T5 where there is a strict limit about going airside unless you have a bp for a flight departing T5.