I don't think there are any bank ATMs at LHR, but there are plenty in Hounslow. Any bank should be fine.
From the linked site, I think only the one at the T3 Post Office may not try to do DCC, although I don't recall seeing an ATM there (but I wasn't looking for one)
Agree - I'd use a bank ATM in Hounslow with contactless on the tube to get there.
A cash only restaurant is a novelty in 2024 - do tell!