I know it wouldn't be professional. But in Parliament, Boris gets to give his statement, Keir replied to it - and Boris then attacked that reply and the Labour party.
Keir isn't allowed to reply to that. He just has to sit there whilst Boris gets to make more replies.
I think Keir just wants to wipe that smug grin off Boris's face and tell him what he thinks of the reply Boris gave. But he can't.