I'm not sure, mil told me she was on her companies cheapest tariff but she has no access to an online account so she will be on their cheapest tariff for non-online account management which is around £250 a year more than what I pay.
People need to research tariffs properly & stop being so apathetic about it.
Problem is that if everyone is on the cheapest tariff then there will only be one tariff and they will just raise the price and those that were paying less will just end up paying more. Unfortunately gas and electric is just one more thing that you need to be online for and shop around regularly for or else you get shafted. Having said that it really is so little work to do that I can't believe that so few people do it. However it's not fair that prepay customers end up paying more and often have less choice.