I remember once at a petrol station I filled my car on a Sunday with petrol but Barclays systems had just gone down so my debit and credit card would not work nor could I use the cashpoint, I had to call my mum to drive to the petrol station with cash, as once you've put petrol in your car you can't take it out again it's not like leaving your shopping at the checkout. On the Monday I took £200 out of my current account and put it in my building society account that I could have used the card in the cash machine if there had been enough money in the account. This was about 15 years ago.
On holiday in Cornwall, about 10 years ago, several of the major banks had a problem with their systems and no card payments or cash withdrawals could me made for about 4 days, luckily we weren't affected, but people were packing up and leaving half way through holidays they had paid for as they had run out of cash and couldn't use their cards to purchase food or get cash out to purchase food so were returning home as they had food in their cupboards and freezers!
More recently we went back to the same site (as we do every year) it was 2021, the shop was taking both cash and card payments but the onsite food and bar were not, generally not a problem, but their card machine was down, the car was packed, we were there with my Dad on father's Day so we booked for a Sunday lunch before leaving. We go to pay and they are like can you come back and pay tomorrow as the card machine is down and we are not taking cash at the card machine will not be replaced untill tomorrow come back and pay tomorrow, it took,a long conversation to get them to understand we would be 190 miles away and back at work tomorrow so could not be back, begrudgingly they eventually took cash.
2022 we were at the same site and had a problem with the car, as we are in the shadow of a hill, mobile and internet is patchy, you can usually get mobile reception on the beach at low tide, but we needed the rac and it was high tide. Reception let us use one of their land lines to call but we needed to pay for new spark plugs and had to pay cash as the rac man did not have any reception for card payments. I always take £200 in cash plus £20 in parking change on holiday just in case. Many do not -we are often 15 miles plus to the nearest cash point!
One of the branches I cover occasionally is covered by a parking app or coin payments but I get no internet reception so have to pay cash, I won't pay in advance as the carpark,is cheap and popular if I get stuck in traffic and am late I won't get a space and will pay twice as much in the multistorey just as far away in the other direction.
I don't usually use cash but always carry it just in case .. If we ever go completely cashless we need to vastly improve the infrastructure to make it work and reliable.