What happens when for structural reasons often or most of time there isn´t "a party with majority"? Or there is such a party, but it doesn´t have "the leader"?The house of commons has control of the finances and it can use that as a powerful weapon to cripple any really unwanted government.
This is the reason behind the custom of picking the PM from the party with a majority. That custom grew out of very practical reason.
The same happened in my own country and many others.
Example of the latter, 1963: Conservative Party did have "a majority", but did NOT have "the leader". 4 likely candidates. Richard Butler, Reginald Maudling, Viscount Hailsham and Earl of Home. It was Elizabeth who picked Earl of Home.
How do Netherlands monarchs act in similar situations?