In London, I used my Chase Sapphire Preferred Visa and Amex SPG for £5 top ups to my Oyster card. I did them within about a minute. Chase translated to $7.17, Amex to $7.26. There were no separate currency conversion fees. Charge was on 2/17.
This seems contrary to recent posts preferring Amex to Visa.
The Visa rate (1.434) approximated the highest inter-bank spot in 2/17, according to
https://www.poundsterlinglive.com/be...-on-2016-02-17. The Amex (1.452) rate was higher, although the pound was dropping at the time.