The T1 Arrivals Lounge is signposted. When you come through into the public area look for the lifts at the back of the hall. Take them up a floor and as you exit them head down the right hand side of the bmi check-in desks. You'll see 'BA Premium Check-In Area' or Zone R signposted. Keep going past these with the check-in on your left hand side and the road outside on your right. The hallway will go down a small slope and it'll be ahead of you.

T4 Arrivals is dead easy. As you get out into the public area turn right and head for the far right hand corner. It is down a short corridor.