If you mean, "if the PM is away then does the Leader of the Opposition have to stay away" then the anwer is no.
If he isn't there he'll be in his allotment or addressing some meeting or other
From Wikipedia:
//If the Prime Minister is away on official business when PMQs is scheduled, their role is usually filled by the Deputy Prime Minister.[21] If this office is not occupied or the Deputy Prime Minister is not available, the next most senior member of the Cabinet will receive questions (such as the First Secretary of State or the Deputy Leader of the government party).//
I think TM would rather be anywhere than PMQ. I have to say I'm surprised at her performance so far at " the ballot box ". She appears nervous, uncomfortable, and shouty.