Thread by @Staedtler

I don't think there ever will be a vote of no confidence, and we have an election by other means, but let's play the game anyway.

Say parliament passes a vote of no confidence. What's next?

There are now two parallel campaigns to try and scratch up the required majority. Both government and opposition will be trying to muster up a majority at the same time. It'll be a mini election to see who can get the fifty percent.

They'll have a fairly accurate idea of which MPs they've got on each side, except for the last few. But they won't know until they vote, and they can't vote until the Queen appoints them prime minister.

And the Queen is only going to call the opposition in to test the confidence of parliament if the current prime minister has given up and either can't get the required majority, or has lost a second vote as well.

Boris Johnson's only winning move is to ask for the extension without waiting till the last minute, and then fight a general election.
Otherwise he risks the Tories kicking him out before he gets a chance to fight an election.