If you take the passageway underneath the Track Transit System and then take the lifts, you'll arrive in a corridor where, to your right, you have the UK border. see screenshot below.




you need to turn right, away from the core of the building and towards the glass walls. You'll see the passageways leading into the jetties and, also, a couple of set of stairs leading towards the ground floor. Choose the one with purple signs for the buses to Terminal 2.