Eurostar - Cheaper to Book in US$
I just made a booking on the Eurostar site.
I got quotes in GBP and USD (UK originating travel.) The USD denominated fare was a little over 5% cheaper than what I got by converting GBP on xe.com, so it was probably more than 5% cheaper because the exchange would make it even costlier to use a USD credit card for a GBP denominated fare.
And, if one pays in GBP there is a 4 GBP surcharge for credit card, and there is no surcharge when paying in USD.