From bilbo's link above, there are no ATMs open at LHR until after security because of covid. So departing passengers can go through security and use an ATM before boarding. As far as I remember, arriving passengers would pass through Immigration and Customs and then directly into an area outside security. Unless I'm misinterpreting something, there would be no working ATMs in that area. Of course, measures taken because of covid will probably change in the near future.

From the linked map there is a working non-Travelex ATM in the arrivals area in Terminal 3, I didn't check the other terminals.

All ATMs at LHR are OK, even Travelex ones - it s the Travelex Currency Exchange counters one should avoid.

At all ATMs be sure to read the screen carefully and decline 'charging in your own currency' (the wording will be different on different machines) - that is where/when the dynamic currency conversion happens. Always get funds in the local currency.