Labour's loss of the working class is not just a recent Brexit-triggered phenomenon. They've slowly been losing ground to the Tories with this group for decades.
1) Gonna look at other shifting gradients too, e.g education & age.
2) The measure used to define working class is share of people in routine and semi-routine jobs. This measure was revised between the 2001 and 2011 censuses, hence the changed horizontal distribution.