It's just a set of instructions. The obligation is entirely contained in the fact that it is an Act of Parliament.
That's why there are no loopholes.
The only objection Boris Johnson has to the Benn Act is that the government did not commission it. In his head, that undermines its authority.
B U T
A law is the law because it went through the stages of parliament and received royal assent. That is what makes it law, not the Prime Minister's approval.
And if he's not careful the court will make him take it in person with everyone watching