It is not illegal.
It's not unconstitutional.
It is inappropriate; the interference is the problem.
with the work of the House of Commons and creates a problem of separation of powers. It is asking the House of Commons to vote on an issue that is at the very same time before the Supreme Court, so there is the real possibility of each one interfering and improperly influencing the work of the other. To be very clear, I do not think that the course of action is illegal or unconstitutional.