In June 2007 the then leader of the Liberal Democrats Menzies Campbell met with the newly promoted Labour Prime Minister Gordon Brown. During the meeting Brown offer two cabinet positions to the Liberal Democrats. Campbell thought it over and refused, citing that it would be unworkable.
I think that this could be considered a good POD. What if Campbell took up Brown's offer?
Having a closer working relationship with the labour party, and sharing a number of leftish points of view, could pave the way for the Liberal Democrats joining Labour in coalition in 2010 instead of the Conservatives.
Fast forward five years; the UK economy is growing again after the worldwide Great Recession, and the two parties manage to win just enough seats to form another coalition. In our TL the Liberals wanted an referendum on having Proportional Representation. Perhaps a more European minded LD leader might consider forcing a referendum joing the Euro.