Originally Posted by
Kdogg
She's hanging around until a new PM is selected so she will be there at least another six weeks.
TL;DR.
To be Prime Minister you have to be the head of a party. In the UK, the head of the party is chosen by dues paying members of that party. Your party has to have the most representatives in Parliament. The head of the party with the majority becomes PM. If there is not majority parties can form collations and share power. (Like the party with more seats get to have the PM and the other gets deputy PM, etc...) There is no direct election to become the Prime Minister. If you are no longer the leader of the party you can't still be PM. The Tories decided they don't want Liz as head of the Tories so she can't be PM either.
The long version for the Parliamentary system. There are differences but this system covers the majority of the democratic world.
Although the Tories and Labour are the biggest parties, the UK has about half a dozen viable ones (and a couple joke ones not including the Tories).
The leader of the party is ultimately chosen by dues paying members of that party. Each party has a different way of narrowing the field that it presents to members to chose from but they have to be an elected member of Parliament.
People vote for a representative to Parliament for their district in an open election. If a party has a majority of the representatives (Member of Parliament - MPs) they can form a government. The leader of the party becomes the Prime Minister and selects the other leadership position in the cabinet. In the UK, they are call frontbenchers. Backbenchers are everyone else in the party. If enough of the MPs loss faith in the PM they can try to remove him/her through a vote of no confidence. This can be done on both the Parliamentary level were all MPs (regardless of party) vote or just within the party. The last three PMs have been removed from leadership of their party. If you don't lead the party you can't be the Prime Minister. The party then looks for a new leader.
In the chance that no single party gets a majority, multiple parties can pool there MPs into a collation and form a government. They figure out a way to split the power. We are seeing this in Italy where the PM resigned, they had a general election but no majority. Giorgia Meloni's Brothers of Italy are trying to form a collation with the League and Forza Italia that will be the most far-right government since Mussolini. It will topple the old government. Same thing happened in Sweden by a narrow margin.
If parties can't from a collation, it's back to another general election. Israel, for example, will have five general elections in the last 2.5 years. That situation is actually worse then the UK but Israel is a smaller player in the world so it doesn't get the same coverage.
When a government has lost the mandate from the people (like this one in the UK) they can call for a general election. The people then decided if it's time for a change or re-up with the status quo. The problem is we are long removed from country over party in the UK.