Charles Kennedy never succumbs to alcoholism, and so the party never goes through its phase of destructive leader-swapping.
Meanwhile, the Blair-Brown spat continues for a few more months than OTL, and Dave Cameron never comes to prominence amongst the Tories.
Charles Kennedy eventually resigns as leader, and the position is given to Vince Cable in 2006.
Cable rules a united and successful party untill his tragic death in 2010. The young, right-leaning market liberals of the party come to the fore, with Chris Huhne serving as leader.
Meanwhile, a single term Cameron premiership, (2009-2013), has been viewed as a failure, and so in the 2013 election, the parliament is hung. The Labour Party under David Milliband and the Lib Dems form the governing coalition. One of the primary demands of the Lib Dems for their co-operation is the introduction of a proportional system of elections.
The coalition collapses in 2015 as Labour cabinet-members get involved in a series of scandals. A new election is called, the first using the new electoral system. The Liberal Democrats, along with their coalition partners, (socialist parties, Scottish parties, etc.), win the election and form the next government.