My husband and I just returned from the UK a few days ago. I tried to use Apple Pay to pay for the Underground at Heathrow, and my husband tried to use Google Pay on his Android phone. Neither worked (both were connected to our US issued American Express cards. I had researched this before and wanted to share our experience in case someone else was going to try soon. We ended up buying Oyster cards. The nice thing with oyster is that you can get the refund in a self-serve machine when you are finished with them. While using our phones would have been easier, the Oyster cards were pretty painless too.