Nearly all utility companies have several different tariffs, the eligibility for which can be dependent on a consumer's usage and income.
I have come across instances of a client finding their supplier has a lower tariff than the one they are currently on; when they approach the company to change to the cheaper tariff, they refuse - until the customer threatens to switch supplier.
It makes me angry too that they can do this legally. OfGem won't do anything about it because it feels that the ability to switch to another supplier in a competitive market allows customers to vote with their custom on such practices.
As far as electricity suppliers are concerned, Ebico is the only one to my knowledge that has only a single tariff for all customers.