In the sphere of my specialist subject (energy, that is; not the random history stories or the doggerel) I am well acquainted with the truism that governments will do whatever it takes to keep the lights on. Indeed, I've often written here in exactly those terms, generally contrasting that primary political imperative with the distant and wholly subordinate 'aspiration' to be on the side of the angels as regards environmental concerns, be they CO2 or air quality or whatever green mantra is trending. As even tinpot dictators know, when the lights go out, the government goes out.
I've never really expanded, though, on just how seriously this is treated in Whitehall contingency planning. Without needing to bang on at great length, just take it from me that they have carefully considered some blood-curdling possibilities. If, for example, a prison loses its power for any length of time, the System is not at all equipped for the consequences. (We don't have watchtowers with machine-gunners in this country.) Likewise, if a major area of the Grid goes down for more than a short while, organised criminals will steal the grid itself: that's what happens elsewhere. The copper is worth a fortune, and it is only the self-protecting nature of a live grid that stops them at the best of times.
Something else I have also mentioned from time to time is that, when the chips are down, the grown-ups just take over, and everyone else is elbowed aside. This can offend a lot of people's finer feelings, concern for propriety and due process etc etc; but there it is.
Taking these two things together, and I am sorry to say the the formal, nay legal 'inevitability' of crashing out in a No Deal Brexit, is somewhat irrelevant. There are enough people in Whitehall who are utterly seized with lights-going-out-style No Deal disaster scenarios that, 'rightly or wrongly', it ain't gonna happen, IMHO.
If Parliament won't vote for something else, we'll just all wake up one morning to an announcement of whatever's been unilaterally decided. And signed. And sealed.
Conspiracy? Unconstitutional? Treasonous? All of the above. Realpolitik can make you feel ill. But there's no way 'they' will let the lights go out. That's just how thing are.