It's a parliamentary system, so the party has to vote in a new leader. We've had similar situations in Canada where the party leader has stepped down, and the party votes in a new leader, who then automatically becomes the prime minister.

I was under the impression the next prime minister would be decided by a leadership contest within the Conservative Party scheduled later this year. As in a similar situation to how Gordon Brown came to power. Although it would be nice to be wrong.

Probably you are right, I don't know too much about the situation, as lots of important facts are always missing from news reports