The United Kingdom does not have a constitution. Tradition. Laws. Rules.
Parliamentary elections in the UK are by district, one member per district. The candidate who gets the most votes wins. Even if there are multiple candidates and the one who gets the most votes gets, say, 38% of the vote.
The elected Members of Parliament (MPs) of the majority party and the members of the party choose the Prime Minister (PM). The Brits are selecting a new Prime Minister because Theresa May, the current PM and leader of the Tory (or Conservative) Party, which has the most Members of Parliament, has resigned. Resigned under pressure from her own party because she could not get the exit plan from the European Union through Parliament. The exit plan she had negotiated with the European Union.
There are rules. Tory MPs interested in being leader of the Party and, therefore, the PM, inform the Chief Party Whip. Ten did. The Tory MPs voted on the ten. The Party had set a minimum of votes to stay in the race. Three were eliminated on June 13. One more withdrew.
Voting continues. Now, the candidate with the fewest votes is eliminated. Until there are two finalists.
There was a public element. Not debates. A series of televised presentations. Candidates are questioned in Parliament.
When the Tories get to two candidates, members of the Tory (or Conservative) Party will choose. Like a national closed Primary. Only voters have been party members for three months prior to the “call for nominations” for a new Prime Minister can vote. They get ballots in the mail and mail their vote back.
We won’t know who wins until the week of July 21.