Thanks all for the replies

I understand all the surrounding thoughts, but I promise that the place I'm going doesn't accept cards and is worth the effort

There is a strip of Pakistani (Indian/South Asian?) restaurants on Bath Rd just a block west of the Hounslow West tube stop that have become must-stop for me whenever I can - Not particularly strong on atmosphere or customer service, but the food is spectacularly flavorful, spicy and delicious.

Current favorite is Khatta Meetha - but I'm still trying others.
And none take cards so far.

I think the problem is that the Travelex (and other private ATMs like the ones I found on Bath Rd.) simply cancel the transaction if you choose not to allow them to do the currency conversion - at the 13.75% markup. I got caught in the airport monopoly.

I see now that there are indeed banks at Hounslow central and a Tesco so maybe I'll try that post office machine next time or head off to a Bank if I have to and hopefully go back to the standard attention to fees etc.