As AFEPPL says above, Apple Pay is live on London public transport (TfL, Transport for London). I use Apple Pay most days on TfL buses - the sensor is on the left side as you get on a bus, so for me it's "on the right side", and it saves me having to get a card out my pocket. On the London Underground (which I use infrequently), the sensors are on the right, which means that if I want to use my watch, I have to stretch over my body which is an uncomfortable manoeuvre, so I use my contactless card. I get about a 95% success rate using Apple Pay on my watch, which is good, but not good enough that I feel completely relaxed about using it, especially in rush hour.
TfL have implemented the same pricing to contactless cards (and Apple Pay) as they do to their proprietary card, Oyster, so there are no financial benefits or disbenefits to using Apple Pay (as long as you use the same payment method over a period of time so that any capping/transfer algorithm knows that you are the same user). So for me it comes down to convenience, and simply it's easy to use on a bus and not on the tube.