I use the ATM at LHR that is right before the escalators when I exit the plane. I get enough cash for cabs and quick meals. I use my CC everywhere else. Never had an issue with no chip (although the cashier at Tesco has to run the card for me). ATM's are the cheapest and easiest way to get cash. I would not count on the taxi's taking credit cards, only a few do and they are few and far between (and never where you need them). Enjoy your lesiure time.