I have access to such a site, but the base currency must be USD. The published USD/GBP rate for 13 February is .688 which matches my actual charges on that date:
American Express SPG (Business)
4.80 = 6.97 = .6887
1.95 = 2.83 = .6890 (1.45)
Just let me know what information you would like me to lookup.
If you use the info on the site you mention and compare directly with Visa and MC rates on their website, do you see consistently that AMEX's rates are the worst on paper?