Under the Benn Act Boris Johnson must send a request for an extension in the form specifically set out as an amendment to the act. But as the UK is still a member of the EU until it leaves, it can veto it's own application for an extension - which is certainly against the spirit of the Benn act but is completely legal. If the veto is exercised at the last minute then the UK paliament would not have time to pass another stupid "Benn" act and the UK would leave by operation of treaty and law.
Unless,of course,Junker The Drunker would,out of the goodness of that great big heart of his,unilaterally grant Britain another extension which,I'll bet,he has the power to do under EU "law".