Hi, all. I previously offered to make a small sacrifice and test a Travelex ATM at Heathrow. Arrived in T2 mid Wednesday. Immediately on walking out of Customs, there is a Travelex stand, with 3 of their ATM's. I used my Schwab card for 20 GBP. It never tried to do a DCC, and the screen and receipt showed "the exchange rate and authorized amount will be determined by your issuer." I checked the exchange rate, the average rate shown online was 1.4927, and my account was charged $29.84. Which is pretty exact. And also in exact ratio with a much larger withdrawal at bank machine an hour later.
So maybe we can trust Travelex ATMs at Heathrow. Which I am guessing can be found at Terminals 1 and 3 also.
PS - By the way, found the walk to the Tube after T2's own Immigration and Customs to be much shorter than I recall for T 1 and 3.