It sounds like war. The thud of the falling pound greeted the prime minister’s tub-thumping speech: it suggested his “great voyage” was destined for the rocks of the hardest of Brexits. Days after proclaiming we would leave with peace, prosperity and friendship, he set out to do battle with those he called our friends and neighbours. Now our ersatz Winston Churchill proclaims he will fight them on our fisheries, fight them on aviation, but above all fight off their filthy regulations. There will be no level playing field on his battleground.

What’s he up to? This marks a sharp U-turn in political strategy. Only days ago, the message was “It’s done. We won. End of story”, as he beat his mini-gong on the stroke of 11pm on Brexit night and Dominic Cummings wept at his own success. At first the order was never to mention the Brexit word again. Get this trade stuff off the front pages and safely hidden away in business sections. Indulge the euphoria of Brexiteer cheers in Parliament Square and let them believe Brexit really is well and truly “done”. Move on instead to trumpet levelling up the north, splurging on bridges and heralding a new era where austerity is over and happy days are here again.

Johnson does not intend to be the fifth Tory prime minister to be brought down by Europhobia

So why ratchet up new confrontation with the European Union? Because before long he will need a great distraction from all the undelivered airy promises. He will need foreigners to blame. This sabre-rattling will have no effect on the imperturbable Michel Barnier, the EU negotiator who knows us and our boorish behaviour all too well by now. Oddly, one reason for this opening salvo is for Boris Johnson to keep his party happy, despite his unassailable majority. A grinning Nigel Farage told Andrew Marr that he was going nowhere: his Brexit party is here to stay, with a new Brexit Watch “insurance policy” ready to “ring the alarm” at any sign of Johnson backtracking. Johnson does not intend to be the fifth Tory prime minister brought down by his party’s Europhobia. That tiger he rode to power still has teeth to turn on him.

Away goes the vain hope that once Johnson was comfortably inside No 10, prudence would prevail. He would quietly settle a deal with Barnier, disguised in new language that would avoid tariffs and barriers. Instead he has told his troops to prepare for maximum border checks as he marches into the identical booby-trap Theresa May set for herself in her disastrous Lancaster house speech: he has built a barrier of red lines that will block his freedom to manoeuvre. His detailed rejection of any alignment with any EU regulations will stymie his own progress.

Cake-and-eat-it time is over. The EU’s position from day one was transparent, its offer crystal clear. No, they haven’t moved any goalposts. The choice is unchanged: the more level playing field rules Britain accepts, the fewer the trade barriers. No alignment, no deal – and fair play must be fairly adjudicated by the European court. That was written into the political agreement Johnson signed. Charles Grant of the Centre for European Reform says the EU is genuinely alarmed that Britain may develop into a deregulated Singapore on its border, unfairly competing by undercutting on standards and social conditions while subsidising our key industries to destroy those of its member states.

Play Video 1:38 'There is no need for a free trade agreement': Boris Johnson outlines UK stance after Brexit – video

Johnson’s riposte to this was preposterous. “The UK will maintain the highest standards in these areas – better, in many respects, than those of the EU – without the compulsion of a treaty.” He listed where the UK outshone the EU’s bare minimum regulations: better on a minimum wage, maternity leave, carbon emissions, live animal transports and more, and of course there is no intention of ever lowering standards. Disingenuous is the polite word: if ours are higher, why not agree to abide by theirs? Because the whole point of Brexit, as he always said, was freedom to diverge when it pleased us – and that can only mean downwards. The EU is right to expect us to lapse unless prevented by treaty.

Johnson may yet regret laying down such rigid red lines. He may wish he hadn’t thrown away subtle compromises. For instance, “take back control” was all he promised, but once established, Britain could have used that control as it wished. We could set our own working time or clean water rules – but they could have converged with the EU’s. If ever we did diverge, the EU could have withdrawn the relevant agreement. The prime minister proclaimed that “British fishing grounds first and foremost for British boats” – but now we command our native waters, we could have been free to let others to fish in exchange for selling our fish into their markets.

Our honour would have been preserved: we would have control as well as deals that benefit us. If that was ever a possibility, Johnson has jettisoned it. Anything that flexible would require the EU to have great trust in us. If Barnier ever contemplated any such deft solution, his dealings with bad-faith Britain – reinforced by Johnson’s speech – will have blown that away.

The Brexit victor had a choice. He could have banked his great “got it done” and still saved the country from needless Brexit damage. But talk to trade experts and they express great alarm that we are headed for trading on thin WTO terms, or what Johnson duplicitously calls an “Australian” deal (Australia doesn’t have a deal with the EU). Anand Menon, professor of European politics at King’s College London, recently thought the odds were 50:50, but now he says, “I’m getting gloomier.” He thinks the barometer is swinging against any agreement as Whitehall prepares for maximum barriers at ports.

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The mystery remains. Johnson had the chance to “heal” and hide Brexit away in a dark cupboard, so why change his mind and opt for a renewed round of anti-EU warfare? Here’s the suspicion. He will need the great distraction of blaming Johnny foreigner for any failure to prosper. He has looked into the Treasury vaults and found no money to match the vaulting ambition of his overblown rhetoric on ending austerity. The Bank of England predicts a miserly 1.1% growth for the next three years: a bad Brexit may drag GDP 7.7% lower.

The chancellor has warned every cabinet minister to find a huge 5% savings from departments that are already threadbare after a decade of cuts. His own chosen current expenditure rules leave no leeway for extra spending: by refusing to raise taxes, austerity is “baked in”, says the Institute for Fiscal Studies.

Johnson can splash cash on capital projects but real levelling up for the north needs gigantic sums. He relies on people misreading big numbers: his £500m to reopen Beeching lines sounds good, but that only buys 25 miles of railway. Will his latest promise of a bridge over every river reprise his Thames garden bridge or that cable car going nowhere? Expect vanity projects before the billions needed for the backlog of repairs in every public service. People will rumble him when his airy optimism and “fantastic” promises crash into his own never-ending austerity. No wonder he reaches for the political distraction of a needless trade war.

• Polly Toynbee is a Guardian columnist