Black cabs haven't been cash only since 2016, its now required to accept card.
Meanwhile, the last time I got a black cab, the card machine was broken, and the driver "helpfully" offered to take a detour to an ATM, with the meter running, on my dime of course.
Of course, only do that at the end of the ride.
They are not allowed to pick up passengers if the machine (or meter, or lots of other things) is broken. If it’s genuinely broken and you still want to pay, you can call the number displayed below the machine and pay over the phone.
It's not as satisfying as yours but my general technique that works everywhere in the world where cabs are competing is: ask them if they take credit cards and their reader is working before you get into the cab. If they say no you find another cab. (In my experience they never say no anyway. But I learned to do this every single time I use a cab anywhere because this scam is so unbelievably common everywhere in the world.)
I got on Uber really early and cashed in big time on those sweet £20 bonuses for EVERY referral.
I'd just stand up at the end of a party and shout "WHO WANTS TO GET HOME FOR FREE" and be set for taxis for the next month or so.
:)
This is a fun way to make tourists feel insecure.
Call them out, where you wait on a foreign police, prove your identity, file a report -- in a non native accent / language -- in the best case...
or in the worst case, where they'll take you where you don't want to go (or other cabs to pickup), potentially drive off in a dangerous way as you forcibly exit the vehicle, and blacklist you from other cars
-- all while you're hoping to make it to a hotel, airport, tour safely and on time.
Most people forget that. Tourist places are a literal gold mine for fraudsters and other kinds of bad people.