Why were the Conservatives (UK) unable to win a majority in 2010?

[DC Al Fine]

To think aloud this has made me realise how much our electoral map has changed in the last 10 years; surely the differences between 2010, 2015, 2017 & 2019 are of the biggest & most erractic set of changes without a change in the franchise?

And as the shifts between elections have not been in any one firm direction each time, well, we can expect this to continue. Of course none of this should really be a surprise: social surveys show that less than half of the electorate has any identification with any party and that those voters who do are more conditional in their support than was once the case. It might now be useful for observers of British politics to pay closer attention to how elections work in Canada...

I for one welcome Britain having routine 20 point swings

[DC Al Fine]

I think we can assume that in the event of an absolute wipeout, UNS might give broadly accurate numbers overall but it would be useless for individual seats. In the event of a Labour landslide, there are seats where the Tories got 60% last time which are much more vulnerable than seats where they got 45%, simply because the Tory voters in the latter are much less likely to ever consider voting Labour than Tory voters in the former.

Yes, the Canadian example confirms this. In wipeouts like the PC's/NDP in 1993 and the Liberals/Bloc Quebecois in 2011, the survivors are a mix of ultra-safe seats, special local factors, and complete flukes.

Examples:

In 1993, one of only two Tory winners was in Saint John, a seat the Tories only won by 5% the previous election. Their candidate was the extremely popular mayor of Saint John and they actually increased their vote share slightly despite a 27% swing against them nationally.

In 2011, Bloc MP Maria Mourani managed to survive despite winning her seat by less than 1% in the previous election, thanks to a peculiar three way split.