I was surprised that the Travelex ATMs at LHR T2 didn't even bother attempting DCC with my Schwab debit card (though they did show 100, 200, etc. GBP presets as opposed to the much smaller amounts that are more realistic for a country with fairly high card acceptance). The receipt even said that the exchange rate would be determined by my issuer.

Haven't run into DCC with credit cards yet but I've only been using contactless thus far.