I can't see it being Boris. There's usually at least one prominent
party member that almost disappears from public view. The idea being,
that if things go badly wrong, they can emerge from the wreckage, and
claim it was nothing to do with them. Whether it's something managed by
the parties or it's a bit of individual opportunism, it happens
regularly. May kept her head down during the referendum, to the extend
that nobody knew what she stood for, only to emerge from the wreckage
without a mark on her.
I suppose Boris did keep his head down, by his standards, for weeks
running up to the vote. It was also often mentioned that Hammond was
nowhere to be seen.
I'm still with Sigmund Freud, I blame the parents.