A guide to using Apple Pay on London Underground:
1) get phone ready by having AP open and on the card you want use.
2) whilst in queue for a barrier, wait until you've got around 2 people in front of you, then fingerprint on your iPhone.
3) you then have 60 secs to wave iPhone on top of the yellow NFC reader on the barrier.
(if some problem holds one of the 2 people ahead of you up, you may gave to repeat steps 2+3 again!)
Et voila, barrier opens and you're through.
4) [sometimes] if exiting the tube at a station without barriers, make sure you do the same as points 2+3 above near the exit from the line you're on (they're usually on a wall or similar near station exit), so you're not significantly overcharged a full maximum day fare.
Simple really, when you know.
Now enjoy the massive queues and over crowding like the rest of us! ;-P
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EDIT:
BTW, TfL (Transport for London) are supposed to be in the process of designing and introducing an Apple Pay-enabled Apple Wallet iOS Oyster card app, and presumably Android one (for those who want to control their pay-as-you-go spending, and/or season Oyster card users), according to news reports a few months ago. But who knows when that's going to arrive...