Thread by @Staedtler

If Johnson goes to the Queen to resign as prime minister he has to recommend someone else.
The difficulty is that normally that would be someone else in his own party

When a prime minister resigns, the governing majority stays the same, so it's usually just a case of hand over to the new leader of that same party. Thatcher - Major. Blair - Brown.
Cameron - May. May- Johnson.

It's very unusual to have a minority prime minister resign without there being a general election.
I mean, what is he resigning as, and why? Presumably he wants to stay party leader. Then what's his excuse to resign as prime minister? He hasn't lost a vote of confidence.

The Queen would ask him - in private, but she would ask - why does he want to resign as prime minister without having been forced by vote of no confidence, yet retain the leadership of his party.

The Queen is not going to accept that crap. She's going to follow protocol. What she's going to say is,
"Mr. Johnson, if you have not lost a vote of no confidence, and you choose to resign, you nominate a replacement from your own party. If you refuse, I will ask the Speaker."

It's because of the confidence part. Until Johnson loses the confidence of parliament he can't resign as prime minister without also resigning as Conservative leader.

And the answer would have to be, to avoid a legal obligation of the office.

He would have to tell the Queen that he wants to remain leader of the Conservative Party, that he has not lost a vote of confidence, but he wants to resign as prime minister to avoid a legal obligation of his office.

The Queen can be quite blunt when she wants to be.
Mr. Johnson. If you can't fulfil the obligations of the office, why do you want to retain the leadership of your party?

If he then says, "I just want to avoid this obligation, then I'm coming back again-"

The next word that comes out of the Queen's mouth will be an expletive.