The devil will be in the detail. Such an initiative conforms to the mantra that choice is the holy grail, and that choice will drive down costs and increase competition. It also saves Mr. Camerons blushes, since he made the announcement in the House without consulting anyone else in the cabinet first.
Offering a simplified tariff structure should make it easier to do some comparison shopping, but how effective such an initiative will be remains to be determined.
Personally, I think it will have little or no effect, and there are other more pressing reasons why the cost of energy has risen so sharply - reasons that will not be addressed by this, mostly cosmetic measure - but I hope I am pleasantly surprised.