Leni
The Conservative Party will elect a new leader and as the Conservative Party cotrols the House of Commons, he will become Prime Minister.
Technically speaking, the Prime Minister is elected by the House of Commons, but because it is assumed that the dominant party will elect their own leader and they have the numbers to do so, in practice, a vote is not called in the House of Commons. The Opposition have every right to call for a vote of no confidence to test the issue if they want to, but won't under normal circumstances.