Well so far the courts reckon it *is* legal, and, as I've voiced elsewhere, I can't disagree with that. But despite all the fuss about John Major doing something similar in 1997, in reality no PM has tested this power for ages. So I don't think there's any harm in checking if it's legal. And ,while it might suit Brexit supporters today for the executive power to prorogue Parliament to be a thing, if the precedent is established that a PM can shut down an inconvenient Parliament whenever they so wish, that's going to backfire against you at some point. Fair enough, you might say, if it is legal -- but, really, my hope is that asking the question and answering it prompts people to try and change the answer for the future.
In the interests of balance, the present situation, whereby the government is trapped into governing without a majority, because the Opposition effectively vetoes calls for an election, is equally intolerable. And also, equally legal -- but, I would hope, it exposes a serious and urgent need to reform or even repeal the Terms of the Fixed Term Parliament Act.