I actually just experienced this and worked out afterwards that if I had gone with the Harrods rate I would have saved about .0070 points after taking into account all the fees etc... (3% Citi). Harrods basically pads their rates by about 3% anyways based on my experience. But then it is my understanding that MC charges 1% on any currency foreign purchase, so I would have gotten a Harrods rate in USD, which is interbank or so +3%, with +1% added for MC fee (since I Citi adds 2% on thier own to make total Citi of 3%)

I took my cc rate when asked though they wont always ask you.

Interesting though that their systems or any system are smart enough to determine that your card is USD based.