David Cameron dies. Wouldn't this make Nick Clegg the prime minister by default?
Your bonus challenge is much more difficult. Hmm.
David Cameron dies barely a week after getting into office, and Nick Clegg becomes PM. In the next three months Clegg tries to show his strength as the Prime Minister, and succeeds insofar that he manages to block most of the Tory Party's proposals including the tuition fee rise. Clegg successfully rides the popularity wave; Tories are unhappy at this.
A leadership election is held in the Tory Party, and Liam Fox becomes the new leader of the Conservatives. Clegg refuses to give up his position as Prime Minister to someone like Fox, but is willing to let Fox become deputy PM. Neither side is willing to budge, and this ends up being the final nail in the coffin for the coalition government. A few more moderate and Europhile Tories defect to the Lib Dems, and a second General Election is scheduled for November 2010.
Up to the Election, Liam Fox says quite a few xenophobic comments, and expresses a desire for the UK to leave the EU. As the Tory Party turns right and anti-EU, they manage to woo back several UKIP supporters, including Nigel Farage who is promised a "significant" government office in the case of a Tory win. However, they alienate a LOT of more centrist and/or Europhile voters. Meanwhile, Andy Burnham, new leader of the Labour Party, isn't doing a great job at attracting centrist voters.
The Tories have also be considered somewhat a failure in government after having many of their policies blocked by the Lib Dems.
The November election results have a fairly large swing from to the Lib Dems. Results: Tories 29.9%, Labour 29.8%, Lib Dems 29.6%. Seats: Con 272, Lab 266, Lib Dem 82.
Many obviously view this as a fairly large travesty, that the Lib Dems would receive the same number of votes as the other parties yet only a fraction of the seats, but more striking is the utter failure of anyone to be able to make a coalition deal. The Lib Dems certainly don't want to get into bed with the Tories again, and a Labour-Tory deal is simply impossible, but eventually Labour and the Lib Dems manage to work out a deal. Clegg successfully pushes Labour to make a lot of concessions, including implementation of proportional representation and leaving Clegg with the Prime Minister job. Labour only accepts as it has no alternative.
By 2012, Clegg has continued to have a largely successful time in government and fulfils most of his pre-election promises. After implementing a proportional representation voting system later this year, he calls for a new general election.
Results: Lib Dem 34%, Tory 30%, Labour 27%. Lib Dems are now the largest party in government. And have the most representation, now that PR is in place. However, they only have a plurality, and now the third different coalition in two years forms: Tory-Labour. They unite on a populist, anti-EU, anti-immigration, economically centrist, platform, sick of spending the last uears subservient to the Lib Dems.
This works, but at the cost of alienating most voters to both parties. As they promote increasingly authoritarian surveillance bills, voters from the extremes of both parties splinter off into UKIP and the Greens, and more liberal voters from the centre switch to Lib Dem.
Three years later, the Tory-Labour coalition utterly collapses, having completely alienated its voters. The government has as a whole performed much worse than the LibDem-led ones, which increases the popularity of the Lib Dems even further. In the 2015 general election, the results are as follows: Lib Dem 51%, Tory 15%, Labour 14%, UKIP 8%, Green 5%, Other 7%.
Yes, this is borderline ASB. It's the best I could do.