Good point. Lammy’s possible as he’s done PMQs when Starmer’s glad-handing abroad; or Darren Jones, one of whose jobs is the new position of Chief Secretary to the Prime Minister
If Starmer resigns but stays on until a new PM is elected, no probem; but if he does so with immediate effect someone will have to fill the interregnum. Just a ‘safe pair of hands’ who has no intention of standing in the election itself. Dropped catches all round?