The Sankei, which keeps track of such measurements reports on April 3rd that:
The rank order of participants in terms of the order of their making a statement and the like is decided in the order of head of state and then chief executive, as well as in accordance with the length of the time since taking office. Prime Minister Aso is the chief executive. His tenure is the shortest among the 20 leaders. For this reason, he is the first among the G-20 leaders to enter the conference hall for the financial summit-related meetings and the last person to leave.Clearly, the Sankei folks use different measuring tools than the British.
He is taking it as a matter of fact, saying, "That is the rule." However, some sources accompanying Aso on his visit to London said with a sigh, "Japan is a major donor, and yet ..."
Later: It should be noted that this is really of another case of Sankei misusing facts to fit their ideology. For those who actually read Japan's constitution, they know that the Emperor not the PM is the head of state. Aso is a head of government, thus of a slightly lower protocol order. For example, if a head of state visits the White House an "official" not "state" dinner is held. The head of state vistor gets a 21-gun salute, while a head of government gets just 19. This article discusses some of the slight, albeit amusing differences.