There is one on the American Express UK website, but it requires a login to access, and I can no longer find a way to trick the site into accepting a USA login (redirects to USA site). The tool displays rates for specific card accounts. Mine is an International Dollar Card, so the rates are displayed in USD with the 3% Foreign Transaction Fee broken out (and excluded from the figures below).
For transactions processed 4 March the USD/GBP rate is .706, the USD/EUR rate is .913 . (We are accustomed to seeing the rates displayed the other way around 1.416 and 1.095 )
Visa is 0.712687 GBP (1.4031) and 0.921482 EUR (1.0852), so Visa is better on both.
MC only goes to 1 March, Barclay's Amex only to 3 March.