Arrived in London this afternoon.
Went on a bus on Kings Road heading west, didn't really note which line.
Tried Apple Pay on my iPhone 6 Plus, where I'd preselected Barclays Arrival Plus.
It showed the UI, showed it accepting my fingerprint and showing "Done" check mark.
But the LED on the pad where people use the Oyster blinked yellow.
Tried twice, both times yellow. So I tapped the Oyster that I also had.
When I bought the Oyster, the TFL clerk said its not supported.
Asked a clerk at an EE mobile shop, also saidd it's not supported and if it works sometimes, it's no guarantee of it working everywhere. But not sure if he was referring to Apple Pay in general or AP as Oyster substitute.
I did successfully make an AP purchase at a Boots store at St. Pancras, using Chase Sapphire Preferred.
Only other possibility is that I put in the Arrival Plus in Apple Pay but never used it so maybe it's not ready to use. But the UI behaved like it works, yet the reader on the bus I tried didn't work.