I just saw DCC for a transaction in front of me at Harrods at LHR. The rate was £52.95 @ 1.1777 USD/GBP and quoted on the register as $62.37. I couldn't see whether the card was a Visa or Mastercard, so I don't know the exchange rate that would have been used. However, the markup would be close to 4% using the current exchange rate. The cashier didn't mention anything, so my guess is that the prompt appeared on the credit card terminal.

I paid with a Visa and used contactless. This avoided DCC automatically for my £35 purchase.