The coalition talks between the Conservatives and the Lib Dems are bringing to the fore what is now known as 'the Disraeli option'. For a good description of it from a Conservative point of view, see Peter Oborne's compelling description in the Observer. For a mediation on how it could benefit Cameron see Sunder Katwala's careful dissection of the multiple options with respect to introducing fair voting, Sunder does not consider the possibility of a Great Repeal Bill securing liberty after 13 years of New Labour as an immediate, progressive legislative outcome that would benefit Britain immediately.
One aspect neither consider is the national dimension, as mapped out in his post below by Gerry Hassan. Shirley Williams has objected to a coalition as this might destroy the Lib Dems north of the border - forseeing their destruction if they put and keep the Tories in office. But as Gerry shows, the Tories are being prevented from enjoying single party rule thanks to the continuing collapse of their influence in Scotland, where Labour is resurgent. If they make First Past the Post and 'winner takes all' politics their sticking point, then this too threatens the Union, for much the best way to achieve it would be to dump Scotland. Cameron could argue that they should agree to put PR to a referendum, because only with it can they have a long-term chance of saving the Union. Or, to put it another way, only coalition government based on PR looks likely to be able to hold together the fissparous units of the UK now diverging from each other strongly.
PS: I forgot to add that all this is a diuscussion of what I dubbed Yellow Toryism