THERE is a famous story of a Tory duchess at lunch in the Savoy Hotel on that fateful day in July 1945 when Labour ousted Winston Churchill from Downing Street, with a whopping majority of 145. Choking on her caviar, the lady is supposed to have exclaimed: “They’ve elected a Labour government – the country will never stand for it!”

On Thursday May 7, the UK witnessed a political shock of similar historic proportions to the 1945 general election. That’s not journalistic hyperbole. On Friday morning, after my own election for East Lothian, I was stopped by a group of young working class women on the main street in Haddington. They knew who I was, loudly proclaimed they had voted SNP, and requested “selfies”. They offered a reason for wanting photos: “This is history”.

North of the Border, even SNP activists were incredulous as the entire post-war edifice of Scottish politics was pulverised into dust. Scottish Labour representation at Westminster was reduced to a single, measly seat in – of all places – bourgeois south Edinburgh. The party’s anointed hard man, Jim Murphy, was ignominiously handed his P45.

This is history indeed, in the sense that things can never be the same again. The trick lies in understanding the dynamics of our new political landscape. The surface phenomena are easy to record. The SNP returned with 56 out of 59 Scottish seats a mere seven months after the independence referendum was supposed to have put the uppity Scots back in their provincial tartan tea caddy. Just as significant, the SNP is now led by a charismatic woman capable of appealing over the heads of the partisan British media to voters in England.

Conclusion: for the first time since pro-home rule Irish nationalists captured swathes of Westminster seats in the 1880s, a party from the Celtic fringe has hijacked the cosy Westminster political agenda. The duopoly of the Conservative and Labour parties cannot recover control without dealing with the SNP. For starters, you can’t run the arcane British parliamentary machinery, with its endless committees and informal speaking rules, unless you talk to the SNP as equals.

What happens if what is now Westminster’s third largest party starts using its numbers to debate reforming parliament, hold civil servants to genuine account, demand errant bankers appear for questioning, or – God forbid – hold up business unless bills get proper scrutiny? What if the SNP has, in boosting its parliamentary representation, actually broken the dominant two-party system beyond repair? Expect a sudden rash of editorials in the “quality” London media suggesting that proportional representation might not be such a bad thing after all.

The historic Rubicon of May 7 extends beyond revolutionising the Westminster system. Scotland voted SNP as a reaction to being told, after voting No in September, that it should be seen but not heard. This patronising attitude was crystalised in Ed Miliband’s refusal to countenance any working agreement with the SNP, as fellow progressive parties, to lock the Tories out of Downing Street. All Miliband succeeded in doing was boosting SNP support among No-voters, guaranteeing Thursday’s landslide.

It is now inconceivable that David Cameron can reject Scottish demands for greater home rule, given that all three mainstream Westminster parties – Tory, Labour and Lib Dem alike – have minimal legitimate authority in Scotland in the wake of May 7. The general election was not a mandate for a second referendum – a point reiterated time after time by Nicola Sturgeon, whatever contrary hares are set running by the battered and bruised Westminster establishment. Nevertheless, the SNP’s electoral success is undoubtedly a mandate for going far beyond the hastily conceived ragbag of new powers contained in the Smith Commission documents.

Put another way, May 7 will go down in the constitutional history books as the moment that the UK was launched on an irrevocable trajectory towards federalism – or bust. If the Westminster system fumbles that move to a federal union of equal British nations, Scotland can legitimately claim it has no recourse but to seek a second referendum.

The constitutional ball is well and truly in David Cameron’s end of the field. Cameron’s opening gambit may well be to offer Scotland fiscal autonomy, in return for termination of the Barnett Formula (a mechanism that matches per capita spending changes across the UK constituent nations). We all know that in present UK economic circumstances a fiscally autonomous Scotland would face a significant budget deficit.

For Scotland to accept fiscal autonomy without inbuilt UK-wide fiscal balancing would be tantamount to economic suicide. However, all federal systems have mechanisms for cross subsidising regions in economic need by regions in surplus. To deny that to Scotland suggests a disingenuous Mr Cameron is hoping to derail any move to Scottish Hole Rule within the UK. Either way, May 7 is a forking of the constitutional road.

BUT we speak only of Scotland. Unexpectedly, May 7 has detonated a political hydrogen bomb under Labour in England as well. Months of glacial opinion polls had pointed to a hung parliament - but with the sporting prospect that a progressive alliance between Labour and a midwife SNP would put Miliband into Downing Street. Instead we have witnessed Miliband and Ed Balls defenestrated and Labour plunged into total – and possibly existential – crisis. Like our apocryphal duchess in the Savoy, the people will never stand for it!

Miliband was quick to blame the SNP for Labour’s defeat, citing a mysterious “upsurge in nationalism” since the No vote last September – in whose absence Labour would have tiptoed into Downing Street.

Of course, this is arithmetic nonsense. Even if all 56 SNP seats had ended up in the Labour column, David Cameron would still have his tiny majority. The real story of the May 7 upset is that Labour was unable to win target seats in its English northern heartlands because working class voters preferred the sleezy, immigrant-bashing populism of Ukip. Rather than take on Farage and his bierkeller candidates, Miliband famously unveiled his own political tombstone. It read: “Controls on immigration”.

As we have proved in Scotland, the way to keep Ukip’s nasty populism in its political box is to fight it every step of the way. But nice Mr Miliband, the son of an immigrant Jewish Marxist, told English working class voters he would “control immigration”. He thereby legitimised blaming immigrants for low wages and pressures on the NHS. Little wonder that those voters decided to vote for the real Ukip populists rather than a Labour Party so scared of Daily Mail and Daily Express headlines that it sold its political soul for votes that never materialised.

ED Miliband I can just about feel sorry for. But not Ed Balls, who had his Portillo moment on Friday morning. Remember that Ed Balls was one of the original architects of New Labour – a bastard political creed that believed giving humungous tax cuts to the “aspirational classes” would win Labour enough extra middle class votes to bribe its way into power.

Balls was the guy who did Gordon Brown’s number crunching in the Treasury. Latterly, Balls was the guy who preached austerity, austerity and more austerity in order to keep those aspiring City bankers on side.

Ed Balls was one of the primary opponents of a deal with the SNP. Ditto Jim Murphy and Douglas Alexander, both unreconstructed members of the New Labour fan club. Time will tell if the May 7 revolution has finally seen the end of the New Labour deviation.

Unfortunately, to my amazement, I heard Jack McConnell try to disinter its political corpse in the early hours of Friday morning. Jack pointed that under Tony Blair, Labour had won three general elections in a row by appealing to the wallets on middle class professionals.

He went on to argue that the sweeping SNP victories on May 7 stretched across every region and demographic of Scotland – implying that this was no left-of-centre upsurge. Ergo: Labour should stick with the New Labour brand rather than adopt an anti-austerity line.

The weakness in Jack McConnell’s position is that the anti-austerity project that the SNP campaigned for in the May 7 election is inherently popular with both the working class and the professional middle class – everyone has stories about not getting adequate care for aged parents and everyone is going to get old. The welfare state is crumbling because of austerity. Most folk in their heart of heats realise we get what we pay for.

One of the few losers on Friday night to bow out with grace and a chuckle was Lib Dem Charles Kennedy, after 32 years at Westminster. He characterised May 7 as the “night of the long sgian-dubhs” – referring to the cull of Scottish Lib Dems and Labour MPs.

Labour in general, and Jim Murphy in particular, seem in complete denial regarding who wielded the political knife. It was not the SNP – it was the Scottish voter. On May 7 the people spoke. It is time to listen to what they said.