A third effect can be that Conservative supporters are less likely to admit it openly as it is considered less "cool". If you would poll people asking what their favorite food is, Big Macs would be underreported as there is a bit of a status stigma against admitting that.

A potential similar effect would be replying with the name of a "cool" party, then not voting at all because they have something else to do, such as work (the election was conducted on a workday, which is a bit weird to me, why not Sunday).