"There is no such thing as a temporary Prime Minister." There could be. Every recent case of a PM leaving office mid-term, the incumbent has been both willing and able to stay in office as a caretaker as the rather lengthy process of choosing a new party leader wends on. (Not even considering 1990, when the incumbent didn't want to go.) But if that doesn't happen, e.g. if Boris is run over by a bus, the caretaker will have to be someone else, thus a temporary.