That's not true the cabinet manual says that negotiations are "freestyle", the incumbent stays until another party commands confidence of the commons but there is no one going first
Explained here:
http://www.theguardian.com/politics...-onto-power-coalition-majority-lib-dem-tories
The incumbent goes first and must resign before the largest opposition party can try to form the government.
http://www.parliament.uk/about/how/elections-and-voting/general/hung-parliament/