It's OK for private companies to go card only, but public utilities and providers of basic necessities should not.
Until recently, I lived in a country where the government have been trying (unsuccessfully) for years to establish a cash-free economy. Shops switched their existing auto tellers to card only and many things like vending machines for public transit tickets, lockers in train stations, etc. were overhauled to accept cards or there would be a mix of machines taking cash only and taking cards only, with the long-term plan of phasing out cash. The result was long queues to pay cash in person, and complaints if the cash-only machines were out of service.
At the start of COVID, this govt put out advice that cash should not be used, to avoid spreading the virus. Some merchants did switch to card only, but it was unpopular - the upscale independent mini-market in my apartment complex did this, but switched back quickly when they lost business to the chain mini-market across the street. My landlord still preferred the rent in cash so he didn't have to pay card processing fees. Small merchants often offer a discount for using cash. Some, like the people who do hairdressing or sewing at home, don’t have any way to process a card. Many people who have a card and are perfectly comfortable using it don’t want to be forced to use it in all situations. I can’t see that going over well in the UK either.
There’s also the escalating issue of fraud and identity theft. I had one of my cards compromised recently - people buying frozen yoghurt with it in Oregon at the same time they were using it in an ATM in Bali - and the bank had to stop the card, freeze the account, transfer the funds to a new account, and issue a new card which took ten days to reach me. I was abroad at the time and would have been really inconvenienced if I didn’t have another bank account with a different bank. Not everyone can afford that. Or what happens if a card is lost or stolen? You can’t have no access to any method of payment while it’s replaced. Not everyone has someone reliable to borrow from. And what happens in a situation like we saw in Kabul recently, where the ATMs were cleaned out and even the banks had no currency? People survived because they had cash hidden at home for an emergency. Yes, it's rare but that's cold comfort if it happens to you.
Finally, it’s inconvenient for people who travel internationally, as many cards will charge a transaction fee and a conversion fee for using the card abroad. At 5 EUR per transaction, this adds up if you’re tapping it for a single bus ticket, buying a snack from a vending machine, giving your change to a homeless person, etc.
All of the issues I've listed COULD be fixed in other ways, but haven't been. Any government move toward eliminating cash or seriously discouraging it would need a full impact assessment and public consultation.