I had to rethink the order of events. Before I tapped on the terminal (the card gets tapped in the front), I couldn't see the top of the screen. However, as the terminal got closer to me and my card went in to tap, that's when I was able to view the screen that stated USD. So the tap must have happened slightly before I was able to see the USD number on the screen. Once I saw the USD come up, I questioned the situation and the agent kept saying "don't worry, everything is in pounds, there will be no issue." I at no point touched the terminal except for tapping the card in the front.

I had another charge the same day with the hotel (same card issuer) and was able to accurately calculate the DCC fee - 4.3%.