It can be as simple as paying for petrol, depending on the operator. Many of these things work contactless with a debit card and this is usually very simple. As I understand it, all new chargers installed since the beginning of last year are required to support contactless payment.
One word of warning. Some of the chargers that take contactless will take an "authorisation charge" on your account when you first present the card. This varies, I have seen £1 from Gridserve and £30 from Applegreen. Someone else reported one that took £45. Do not panic, you will not be charged that, but if your attempt to charge fails for some reason the debit will sit in your "pending transactions" for a few days before disappearing. I accumulated £90 of such debits with Applegreen before going away and getting connected to a nearby Gridserve charger. It wasn't a big deal, I didn't have to apply for my money back or anything, but if I hadn't had enough credit in my account to allow £90 to be frozen over a weekend it could have been difficult. As far as I can make out, if you charge successfully this holding charge disappears immediately to be replaced by a real debit for the amount of electricity you bought, but if the charge fails the authorisation charge takes some time to vanish. It seems that they are checking that you actually have the money in your account to pay for what you're about to take from the charger, but this is by no means a universal practice. And Gridserve only take a £1 charge, presumably just to check that the card/account is real.
However, there are still chargers around that don't take debit cards. I encountered this in relation to an outfit called Blink Charging, who run type 2 chargers I knew I would want to use. I was fortunate that I checked this out before I left home, because it wasn't simple. They wanted customers to have their own RFID card, but there wasn't time for anything to be posted to me. A fair bit of surfing got me to the place where I could download their app, and this wasn't easy either. The page to join as a guest didn't work because of a glitch in the phone number entry. To become a member you had to have a Google account, and at first this simply didn't work and dissolved in electronic gibberish. I emailed the company, got an automated reply, but no actual help. I remembered Google had two email addresses for me. I had to delete the app completely and start again with the other email, but that worked and I got registered. Except the app didn't have the MG4 as an option to enter your vehicle. (I knew I was OK though because my friend who lives right beside the chargers said she saw an MG4 charging there.)
When I got to the chargers I was so glad I already had the app, because the sun was so strong I could barely read either the charger screen or my phone. Downloading the app by the roadside would have been a nightmare. The charger kept asking for a card, but I went into the app (hiding from the sun inside the car boot) and found how to connect. I did phone the helpline at one point, mainly because the strong sun had concealed the fact that charging had started, and the girl who answered was helpful. More than I can say for the email. Later I got an email asking me to rate the service I had when I emailed them for help at the beginning. I said I got no help at all, and never heard another word. But the chargers did work from the app.
I'm telling you this to let you realise what can happen, and the advisability of getting at least the app and preferably the RFID card you need for a charger you know you're going to need in good time. Don't rely on rocking up to a charger like that and hoping you can make it work. I've seen people online brandishing fistfuls of RFID cards that make them look like Las Vegas croupiers. I got the info about which chargers took debit cards from ZapMaps, although they might not be the best they were right in my case.
Another thing you should do is take your car to a nearby public charger or two and practise, when you have the time to fiddle around, and it's not a disaster if you don't succeed. (I did this, but only on one with tethered connectors. I ended up tearing the wrapping from my type 2 cable with my teeth on the pavement beside the Blink chargers.)
. I got the best and worst of public EV charging in one visit!
are you sure you wasn`t in a Sainsburys car park { Free charging God bless em }
, cause that's the kind of guy I am, 
