When evaluating the role of the PM, much more needs to be taken into consideration than simply the powers afforded to him by the British Constitution. The effect of the media, popularity and events are also crucial.
In this article, Iain Watson for the BBC explores whether Miliband did enough at the Labour Party conference to show that he could indeed lead the country. Look here for both the importance of policy put forward as well as more intangible factors such as his leadership qualities. Do not fall into the trap of thinking the success and power of a Prime Minister can be quantified simply by what he is purported able to do.