We’ve been living in London for the last couple of years and while the “no cash - it’s all tap to pay” advice is valid, it comes with a few caveats.
Bathrooms: as someone has mentioned.
Food stalls: at smaller food festivals/markets (like the Saturday food bash at Duke of York Square) there are more than a few vendors who prefer cash, and will make that plain - sometimes with a sign, and sometimes by just asking you when you pay. They have a card reader 99% of the time, but this is a real phenomenon.
Black cabs: I’d say a minimum of 20% of black cabs have a sign posted saying they “will accept” cards but “prefer cash,” or some variation of that. Again, 99% of the time they do have a card reader, but prefer cash. Additionally, paying cash can often score you a small discount, i.e., the driver will round down so you can pay with a note and he won’t have to break it to make change. :-)
This is all just to say that if you do get cash out of the ATM, you will likely be able to use it. Just… don’t withdraw very much.