What's interesting is that if suppose that Unsure don't vote, then the Conservatives have effectively capped out their vote. YouGov polling has them on 42%, the % who'd consider voting for them excluding Unsure is 44% - only 2% of the electorate who currently aren't voting the Conservatives would even consider voting for them.

Compare this to Labour, where YouGOV has them polling at 26%, but their "consider" score is 35% - about 9% of the electorate who currently aren't voting Labour would at least consider voting for them. That's some clear room for expansion, but... even if Labour capped out, they wouldn't be beating the Conservatives unless those who considered voting Conservative were split between another party.

I think how well UKIP did in 2015 actually disguised the underlying unpopularity of the Labour party by depressing the Conservative vote. Same in 2010 with the Lib Dems. People blame Corbyn and he clearly has a part to play, but I think it is clear there is a much deeper malaise in the Labour Party, stemming back probably to the 2008 recession. People have long memories.