He stands in if the Prime Minister is overseas or indisposed - much the same as the Vice President in the United States.
London doesn't have a Prime Minister. The Deputy Prime Minister of the UK is Nick Clegg.
John Prescott.
Gordon Brown was Prime Minister of the UK in 2009. Deputy Prime Minister is not a permanent position in the UK Government - it is a position which is created depending on the wishes of the incumbent Prime Minister. In 2009 there was no Deputy Prime Minister.
Abdyrakhman Mamataliev is the Deputy Prime Minister for Kyrgyzstan.
nick clegg is the deputy prime minister
Guatemala does not have an office of Deputy Prime Minister.
The deputy Prime Minister is a post that is the gift of the Prime Minister and the UK currently has no DPM, the last DPM was John Prescott, who was in office while Tony Blair was PM.
Deputy Prime Minister will govern the country in absence of the Prime Minister. eg: if the Prime Minister is seriously sick or incapacitated.
Anatoliy Kalinin is the Deputy Prime Minister for Belarus.
Tsvetlin Yovchev is the Deputy Prime Minister for Bulgaria.
Amadou Ali is the Deputy Prime Minister for Cameroon.
Branko Grcic is the Deputy Prime Minister for Croatia.