A lot of people don't know this and even if they do there's usually a minimum services will accept before they leave you alone. For example when I missed my water bill payments Thames water doubled my payments to £60 a month. I called them saying that the original £30 is all they're getting but they kept threatening me so I eventually agreed £40 a month with them. So you're right that they had to accept a payment plan but I still had my monthly bill increase by £10 just for missing a payment.

Pre payment is a rip off, charging more to the people who have the least amount of money

You're not wrong, it's an example of a poor tax which I accept but I also accept that not everyone else does. I prefer to pay more rather than get harassed over bills just like how not everyone wants to pay their car insurance upfront to get it cheaper. At the end of the day different methods work for different people.