So what’s bothering them? Why the sour faces? Surely this is the moment of their greatest triumph? Surging ahead of Labour in the polls. The snarling Right-wing press firmly on side. Only a tiny number of recalcitrant Conservative MPs. And the Article 50 letter, triggering the final steps towards the Promised Land of Brexit, tantalisingly close.

Yet, in their TV interviews this weekend, David Davis looked crumpled and grumpy, Liam Fox blinked like a startled rabbit and Boris Johnson mumbled even more implausibly than usual about how splendiferous Brexit will be. Meanwhile, the papers were full of simmering bile between party apparatchiks in No 10 and officials in the Treasury as they tried to duck the blame for the OmNICshambles Budget.

It is one of the most intriguing paradoxes of this government: its dominance of British politics is secure yet it acts with nervous insecurity; it possesses unrivalled strength yet it feels much weaker than it seems. Some of that derives from the founding flaw of this government — it has a PM who was never elected by anyone, has no legitimacy of her own, while its party won the support of only a quarter of all eligible voters at the last election. Thoughtful Conservatives know they have landed on their feet through luck and guile, not because of widespread affection from the British people.

But I believe there is another reason too: of all the possible Brexit scenarios — bad deal, better deal, no deal at all — it is dawning on sensible folk in Whitehall that the one scenario that will most definitely not occur is the one they promised to the country. In the week when Theresa May is likely to pop that Article 50 letter in the post, it is worth recalling what we were promised. Boris Johnson has said “we will trade as much as ever before, if not more”; David Davis told Parliament that we will get “the exact same benefits as we have now”; Theresa May promised to settle the terms of the divorce and all the details of our new relationship with the EU, including full ratification, within two years; Michael Gove pledged there will be “no change to the border” in Ireland; and ministers announced that a raft of new trade deals around the world will be of greater value to us than our current trade with the EU.

I struggle to be polite about this mountainous pile of self-deluded piffle. Before I receive the usual barrage of condemnation from fanatical Brexiteers, let me give you three simple reasons why the Government is condemned to break its word on Brexit in a way that will make the £350-million-for-the-NHS lie, or the National Insurance shambles, pale in comparison:

First, geography still matters. It perplexes me that ministers persist in telling the British people that distance does not matter for trade, especially in goods. If you listen to Messrs Johnson, Gove, Fox and Davis you’d think the fact that we are tectonically located next to Europe’s land mass was of no significance. They blather about the great potential for new deals with Australia, Canada, India, New Zealand and the rest of the “Anglosphere” as if the planet had suddenly become as small as SW1. But there’s a reason why our trade is almost 39 times greater with Ireland than it is with New Zealand. Ireland is next door and New Zealand is on the other side of the globe. There’s a reason why Mexico and Canada trade more with the US than we do — they’re neighbours. Even if we doubled all our trade with the “Anglosphere”, including the mighty US, we still wouldn’t trade with them as much as we currently do with the EU.

Nothing that the Brexiteers can say, wish or do over the coming years can abolish the simple fact of geography that makes trade with our European neighbours so hugely important.

Second, the rule of law. Again, if you listen to leading Brexiteers you’d be forgiven for thinking that the only thing that matters is whether British goods will continue to enjoy “tariff-free” access to the EU’s single market. But the single market — devised by Margaret Thatcher — has little to do with tariffs and all to do with removing the plethora of rules, standards, qualifications and norms that govern everything from phytosanitary standards for beef exports to mobile phone roaming charges. The genius of the single market is that it replaces 28 fiddly rules with one. In other words, it simplifies rather than duplicates red tape. But the system only works if everyone abides by the same rules and if there is a court — the European Court of Justice (ECJ) — to settle disputes.

In declaring a fatwa against any British adherence to the ECJ, Theresa May has struck a death knell for continued British participation in the single market. Unless she makes a U-turn on the ECJ, or the Brexit deal invents another similar court, there isn’t the remotest chance that Johnson or Davis’s pledges that we will continue to trade on the same or better basis with the EU will materialise. They refuse to admit it but it is logically impossible to have frictionless access to a marketplace of rules if we’re not prepared to abide by the rules of that marketplace.

Third, political narcissism. The self-absorption of the Brexit debates in Parliament over recent months confirms that the fate of our country has now become victim to an introverted argument within the Conservative Party, as if no one else matters. Conservative Brexit MPs and their sponsors in the Brexit press pontificate to each other as if the Brexit talks will be settled among them. Michael Gove loftily declares that we shouldn’t negotiate at all.

Yet we will have to — with 27 other governments and parliaments. No wonder this government appears nervy and brittle. It is about to collide with reality.