A lot of businesses in the UK/Europe do not accept American Express due to higher fees. You would be better off getting a different card preferably that supports contactless payments so you could use it on the tube and buses instead of an Oyster card (remember to touch it on the way out as well).
Upvote and reply, because Revolut is a dream. Prepaid so fraud is minimum risk (no possible overdraft), top up via mobile app, disable the card when you're not using it. It's so convenient that I use it to buy from shady sites (game keys, in game currency etc) and just disable the card once the purchase is complete. I can't believe it's still free.
https://tfl.gov.uk/fares-and-payments/contactless