It’s not spoken about much in terms of ‘sliding doors’ moments, but Clegg’s decision to take such an active role in the coalition set the LibDems back 20 years and destroyed all the momentum they had gained prior to that election.I'd say that perhaps the bigger thing was the hysteria about tuition fees among the well to do middle classes, as that sank Clegg and the LDs ability to temper the Tories in government, leaving Cameron with the only people to pander to was the right of his party, hence why he promised the referendum in the next election.
Think things would have taken a different direction for British politics if they had taken much more of a back seat role or played no part at all.