I cannot imagine in this day and age that someone would be selected as PM without also being an MP. Now, I can imagine them losing their MP seat during their term as PM and staying on.
The Tories and Labour know that the way the PM is selected isn't exactly Democratic, and they know that if they were to select someone who hasn't had a single vote that would be politically disastrous and likely result in a change to the way PMs are even selected.
The UK's whole political system only works because of mass voter apathy and ignorance (see the AV vote for examples).